The Ampharieion was situated in a valley about 2.22km from the town of Oropos. Situated upon an important border, the ownership was often contested - and in a way this contributed to the making this sanctuary as renowned as it was. (For in order to gain control of the sanctuary - what better way than to try and shower it with more gifts than your enemy?)
The sanctuary, because of its being positioned somewhere strategically important, continuously switched between the hands of the Thebans and the Athenians. Pausanias  mentions how Oropos originally belonged to Boeotia and following the constant changing of hands, was finally given to Athens by Philip after he took control of Thebes.
Pausanias records the sanctuary as being 12 stades away from the coastal city, and that upon the site was both a temple and a white marble statue of Amphiaraus himself. He also goes into depth about the gods featured upon the altar (see next section). Furthermore he informs of of the 'Spring of Amphiaraus' situated near the temple which they used for neither sacrifice, purification or Lustral water. 
We have the Archaeological evidence of a theatre, stoa, altar and curved standing area. The curved standing area was likely where people would stand to watch the sacrifices.
According to Parker there is little precise evidence for use of this sacred site within the fifth century apart from two small altars and an adjacent 'theatre'. Rhodes and Osborne suggest that in the early 4th century there was a small temple, a fountain, a theatral area and a sleeping area likely made of wood (since no trace of it survives), all of which were to the west of the later sanctuary. (As is shown in fig.1, the larger temple of Amphiaraus being added on in later years). As the Athenians sought to embellish the sanctuary they later added said larger temple, a stoa (110m in length), a stadium (for the sports in the Amphiaraia) and a theatre. And continued to embellish it after they regained control of it in the 330's. Inscriptions  found at the Amphiareion highlight this. The first of the two  refers to the installation of a water drain to control the flow of water during storms. The second  honours Pythias for the construction of the fountain, the drains and underground conduits. By the end of the 4th century therewas a small altar, theatre-like steps (which were later removed and replaced with a small theatre), stoas (for people to sleep in), and two bathhouses (mens and womens).
It is notable that you descend into the valley, similar to the way in which Amphiaraus descended into the eart.
The key god worshipped at the Amphiareion was the hero Amphiaraus who was known to not only have prophetic abilities, but also healing abilities. Originally Amphiaraus was a king of Argos, favoured by both Apollo and Zeus (from whom he had recieved his oracular power). He was tricked by his wife into participating in the battle between Polynices and Eteocles as one of the Seven against Thebes.
Amphiaraus had originally not intended to take part in the march against Thebes, having divined that the sole surviver would be Adrastos, but Polynices went behind his back and bribed Amphiaraus' wife, Eriphyle with a necklace, who thus persuaded her husband to proceed on the venture. During the battle, whilst he was fleeing up the river Ismenos he and his charioteer were swallowed up in a chasm created by Zeus hurling a lightning bolt in order to save him from death, and thus he was made a god.  On account of this underworld ties, he was worshipped as a chthonic deity. Although retelling the story sightly differently, Pausanias also agrees that he was, in fact, swallowed up by the earth. 
Pausanias also provides, in his 'Description of Greece' a detailed description of what was upon the altar (and thus to whom people were meant to sacrifice to as part of their rituals). From his description we can asecretain that it showed different scenes on different parts; on one part Heracles, Zeus and Apollo Healer reside; on another heroes and their wives; on another Hestia, Hermes, Amphiaraus and the children of Amphilocus; on another Aphrodite, Panacea, Iaso, Health and Athena Healer; and on the final one nymphs, Pan and the rivers Acheloüs and Cephisus. He also points out how Amphiaraus' son Alcmaeon is notably not worshipped, as Pausanias believes, for the way he treated Eriphyle and his getting revenge for his father. Whereas if we look back to Apollodorus early on we see that Amphairaus had, in fact, instructed his sons to kill their mother when they came of age (for her betrayal). This is addressed in Ovid's 'Metamorphoses' as he refers to it, in Themis' prophecy as a duty which is also a crime. 
Iaso (goddes of recuperation), Panacea (goddess of remedies), Apollo Healer, Athena Healer, Health are all gods related to the healing process, and so to ensure being cured properly it was wise to include them in the sacrifices. Iaso (in the fifth century) according to the Edelsteins, appears even to be the daughter to Amphiaraus within the testimonies of Asclepius. (Testimony 235) Iaso and Panacea are both linked to Asclepius alos, but are often forgotten when in comparison to their sister, Hygeia.
Heracles was also a god linked with health, but especially sports and strength. Since this site was a site for games, it was wise for him to be included amongst those being sacrificed to.
The nymphs, Pan and the river gods are all related to nature and likely to be related to the very natural setting in which the sanctuary was situated. (Pan in particular was known for slopes, and the sanctuary itself was situated on the slope of the valley).
"One who has come to consult Amphiaraus is wont first to purify himself. The mode of puficiation is to sacrifice to the god, and they sacrifice not only to him but also to all those whose names are on the altar. And when all these things have been first done, they sacrifice a ram, and, spreading the skin under them, go to sleep and await enlightenment in a dream." 
Pausanias' passage above quaintly explains the generic process of incubation at the sanctuary. However the presence of baths, a temple and the presence of a priest bandaging a man's hand in a relief (the one referred to as the Archinos relief) , would suggest that there is more to the rituals than merely the sacrificial stage. A similar process is required for Asclepius in that supplicants would have to bathe and offer sacrifice to him before incubation.
However the idea of sleeping upon the skin of the animal seems to be important and there is some evidence, according to Rhodes and Osborne, that the skins acquired by the sanctuary could be sold on. They highilght how the providsions that the skins should be sacred was chiselled out without adding a new clause. Further more looking at an inscription on the acropolis (ca.335 BC), it suggests that the sacred items of either Amphairaus or Ascelpius could be used to make money and for that money to be used upon the upkeep of the sanctuary. 
Anyone could sacrifice should the priests not be present, and make their own prayers, but if the priest was present it was preferable for him to do so - and for the shoulder of the sacrifical animal to be given to him (as it would be upon public occasions too). The priest would always preside over public (or festive) sacrifices.
The fact that around the altar there is a curved standing area for witnesses, would suggest that people were encouraged to watch said sacrifices - and combined with the theatre and the games at the Amphiaraia, the temple, then, was a very public place and sacrifices were part of the attraction.
Any animal could be sacrificed but it was only the ram skin, as mentioned above, that could be used for the incubation process (in which Amphiaraus was meant to provide the answer to their ailment by way of a dream).
The bases of dedicatory statues survive to this day but little else of the statues or relieves themselves, including;
- SEG 31 457 (A fragment of a base)
- SEG 31 433 (Fragment of a painted pinax)
- SEG 31 432 (A base with kymation) Dedicated by Chrysion, after 322 BC
- SEG 31 429 (Fragment of the lower part of a relief, showing a leg.) Dedicated before 338 BC
- SEG 31 435 (Upper part of a base) Dedication by the Ephebes ca. 4th BC
- SEG 31 440 (Base) Dedication to Amphiaraus, ca. 300 BC
- SEG 31 431 (Two joining fragments of a base) 338-322 BC (IG VII 490)
- SEG 31 454 (Three fragments of a base) Dedication to Amphiaraus, 1st BC
- SEG 31 434 (Three fragments of a relief with kymation) Dedication to Amphiaraus 4th BC
- SEG 31 452 (Two marble fragments) ca. 200 BC (IG VII 403 and 501)
- SEG 31 437 (Statue base) Dedication of a statue of Glaukos 4th BC
- SEG 31 444 (A small base) Dedication to Amphiaraus, ca. 300-250 BC
- SEG 31 443 (Two blue marble fragments) Dedication to Amphiaraus, ca. 4th-3rd BC
- SEG 31 442 (Right part of a limestone base) Dedication to Amphiaraus, ca. 4th-3rd BC
- Seg 31 439 (Fragment of a marble base) Dedication to Amphiaraus and Hygieia, ca. 300 BC
- SEG 31 445 (Base)Decdication to Amphiaraus, ca. 300-250 BC
- SEG 41 447 (Dedication on a sundial) Roman Imperial Period
- SEG 28 462 (Dedication of an Athenian Bouletai) ca. 328-7 BC
- SEG 31 441 (Curving stone base, concave on front as if the base of a seat)Dedication to Amphiaraus, ca. 4th-3rd BC
- SEG 57 446 (Honorific statues in the Amphiareion) (public documents)
The range of dates suggests that the sanctuary survived from the 4th century right through to the Roman Imperial Period and that a range of Greeks and Romans honoured the god. Alongside these dedications there were also numerous stelai found bearing decrees of the Athenian people, depicting their continuous involvment in the affair of the sanctuary - ranging from honouring, to games, to repair works. Furthermore there was also a rather troublesome herm found which until recently had been considered an anomaly age wise but was more recently dated to 470-50 BC. 
The key festival we know of is the Amphiaraia, which we know the Athenians expanded ca. 330-20; they added a procession, atheltic and equestrians competitions. In an inscription mentioned in Rules and Regulations  the earliest reference to this is found, though it doesn't name the Amphiaraia explicitly. (line 34) An inscription  found at the sanctuary of Apollo informs that it is a quadrennial festival (occuring every 4-6 years) and that Athenians were prominent in the hosting of the festival as well as the sacrifices. Upon this decree a particularly prominent Athenian is honoured - Phanodemos of Thymaitadai.
Another inscription  (329/8 BC) specifically mentions that there were athletic and equestrian competitions at the festival. It also provides a list of names whom they're honouring for contributing funds for said festival. Phandemos of Thymaitadai is once again amongst those being honoured, the man who, allegedly proposed the legislation for the festival, as well as contributing to it greatly.
The inscription  upon yet another stele found at the Amphiareion (ca. 332/1 BC) Phandemos proposes the dedication of a gold crown of 1000 drachmas to the god Amphiaraos. And this is taken up in the decrees which follow it at the Amphiaraion in that it then becomes the thing to do when honouring people for their services to Amphiaraion or the Amphiaraia.
"When a man had been cured of a disease through a response the custom is to throw silver and coined gold into the spring, for by this way they say that Amphiaraus rose up after he had become a god." 
Whilst the sanctuary did hold asylia (protection from persecution, plundering, etc.) it did not necessarily guarantee saftey from pursuit for a misdeed. They required you to hang a wooden tablet bearing your name at the entrance of the temple before you were able to enter the Amphiareion. This was likely to discourage the terms of asylia from being abused and to keep only the right sort of clients using their temple (for sanctuaries were run a little like businesses). 
Rules and Regulations
A marble stele broken into three pieces was found at the Amphiareion and provides the following rules and regulations :
- The priest must be present from the end of winter until the ploughing season
- The priest must not be absent for more than three days
- The priests must remain in the sanctuary not less than ten days each month
- If anyone commits an offence in the sanctuary the priest may inflict a punishment of upto five drachmas (and if the man pays , the priest is to be present when it is deposited in the treasury)
- The priests is to give judgement if anyone is privately wronged in the sanctuary of upto three drachmas, but larger cases are to be taken up in places where the laws state
- Whoever comes to be cured has to pay at least 9 obols of silver and deposit them in the treasury in the presence of the keeper of the temple
- The priest is to make prayers over the offerings and place them on the altar if present
- If the priest is not present then the person making the sacrifice is to make the prayers
- At public sacrifices the priest is to make the prayers
- The skin of every sacrificed animal is sacred
- Any animal may be sacrificed
- No taking meat outside the boundary of the sanctuary (so no take-away)
- Those who sacrifice are to give the priest of each sacrificial animal
- On festival occasions the priest recieves the shoulder of each of the victims at the public sacrifices (not private)
- The keeper of the temple is to record the name of whoever incubates (sleeps at the temple overnight) after he deposits the money - both his personal name and the name of his city
- The board it is written on is to be displayed in the temple
- Men and women are to sleep separately (men on the east of the altar, women on the west)
A stele mentioned above  (332-1 BC), was found at the Amphiareion, in which the Athenians honour Amphiaraus with the following;
- a gold crown of 1,000 drachmas
- a herald to annouce that the Athenian people crown the god with such a crown
- it shall be dedicated to the sanctuary once made on behalf of the health of the people of Athens
- the prytany shall inscribe all this on a stone stele and stand it in the sanctuary (for which 20 drachmas from the treasury of the people shall be given)
Seeing as many of the following decrees all end in the same way, they werethus preserved at the Amphiaraion upon said steles. And furthermore, as mentioned above, this symbolises an honour that was later recycled when honouring people for their services to the sanctuary of Amphiaraus.
On the border between Boeotia and Attica, Oropos was a strategic position and often switched hands between the Athenians and the Thebans. Thucydides  refers to the people of Oropos in 431 BC as being dependants of Athens in Boeotian territory. In 427-8BC he also refers to Oropus being in the hands of the Boeotians.  In 424/3 Thucydides records the Athenians pulling out of Boeotia but remaining on the "frontier at Oropos"  where a battle ensues (which the Athenians ulitmately lose) and the Athenians retreat from Oropos. In 411 Thucydides informs us that in the winter of that year the Boeotians had to recapture Oropos from the Athenians which they had gained at some point "as a result of treachery" . He goes on to inform us of the strategic importance of Oropos for Athens in any manoeuvres against Eretria and Euboea in the same section. After this they remained under Theban control until the King's Peace, whereafter they were indepedent. This independance lasted all of around ten years before it was taken over by Athens ca.374 (as they preferred not to be forced into a Boeotian League)  until 366 B.C. Oropus was taken by Thebes yet again . Eventually it was given to the care of the Athenians by Philip following his defeating the Thebans. It was not uncommon for poleis to fight and argue over who certain sanctuaries belonged to, and since this was the most well-known sanctuary of Amphiaraus, it is not suprising that it was fought over - and that Athens seemed keen to participate.
Despite all of this (or perhaps because of it) the sanctuary continued to prosper well into the Roman imperial era. Though it is important to note that despite being controlled by Athens, it was never made into an attic deme.
Who used the site, and where did they come from?
Aside from Pausanias' claim that the sanctuary was originally in the hands of the Thebans, the origin of the sanctuary and even the cult is questionable. Some would argue that the cult was originally from Thebes but following the Thebans choosing to have Amphiaraus as an ally as opposed to an oracle  and that it was transferred over to Oropus - bringing with it the idea that whilst Amphiaraus had sunk into the earth at Thebes (courtesy of Zeus), he had remerged in the sacred spring at the Amphiareion at Oropus , especially since Strabo suggests as much - naming Knopia as the original site (a site likely in Thebes, but as of yet unidentified) . Parker suggests that this similarity with Asclepius shows how mythical healers weren't retstricted geographically like other heroes. The Thebans being unable to consult their oracle would suggest why the Athenians seemed to play more of a role at this sacred site. Although it may have been more of a political manoeuvre since Oropus was so close to the Theban border.
Others would argue, from the archaeological evidence (namely 2 reliefs commemorating victory in games around 400 BC) as well as the terminus ante quem provided by Aristophanes' Amphiaraus (414 BC), that the site was founded in the midst of the Athenian period and was perhaps the Athenians trying to assert their dominance in a disputed area (and thus all the attention they lavished it with through the years following their regaining it from the Thebans. However another view is that the worship of Amphiaraus was originally founded by the Oropans alone.  However Pausanias is a later source and so not as reliable in some ways. The different accounts, evidence, and lack of Herodotus being vague about the whereabouts of the temple of Amphiaraus in his Histories, leads to the afore mentioned confusion as to where the cult sprung from and who would originally have used it.
Select Site Bibliography
Fig.1 Nefasdicere at en.wikipedia ((http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 or CC BY 2.5), from Wikimedia Commons
Fig.2 Nefasdicere at en.wikipedia ((http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 or CC BY 2.5), from Wikimedia Commons
Fig.3 Nefasdicere at en.wikipedia ((http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 or CC BY 2.5), from Wikimedia Commons
Fig.4 Nefasdicere at en.wikipedia ((http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 or CC BY 2.5), from Wikimedia Commons
Fig.5 Nefasdicere at en.wikipedia ((http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 or CC BY 2.5), from Wikimedia Commons
Apollodorus, The Library of Greek Mythology (trans. Robin Hard. Oxford University Press 2008)
Pausanias, Description of Greece Vol. 1 Books 1-2 (trans. W.H.S. Jones. Harvard University Press 1918)
Ovid, Metamorphoses (trans. D. Raeburn. Penguin Books 2004)
Thucydides, History of the Peloponnesian War (trans. R.Warner. Penguin Books 1954)
Herodotus, The Histories (trans. T.Holland. Penguin Books 2013)
Strabo, Geography Volume IV: Books 8-9 (trans. H.L.Jones. Harvard University Press 1927)
Isocrates, Evagoras. Helen. Busiris. Plataicus. Concerning the Team of Horses. Trapeziticus. Against Callimachus. Aegineticus. Against Lochites. Against Euthynus. Letters (Trans. L.R.V.Hook. Harvard University Press, 1945)
Aeschines. Speeches (Trans. C.D.Adams. Harvard University Press, 1919)
Greek Historical Inscirptions 404-323BC, eds. P.J.Rhodes and R. Osborne (Oxford University Press, Oxford, New York, 2003)
Sanctuaries and the Sacred in the Ancient Greek World, J. Pedley (Cambridge University Press, Cambridge, New York, 2006)
Asclepius, Collection and Interpretation of the Testimonies, E.J.Edelstein and L.Edelstein (The John Hopkins University Press, London, 1945)
Athenian Religion, a History, R.Parker (Oxford University Press, Oxford, 1996)
Greek Sanctuaries, new approaches, eds. N.Marinatos and R.Hèagg (Routledge, London, New York, 1995)
Archaic and Classical Greek Art, R.Osborne (Oxford University Press, Oxford, New York 1998)
Supplementum Epigraphicum Graecum via http://referenceworks.brillonline.com accessed 29/01/15
Attic Inscriptions Online https://www.atticinscriptions.com/ accessed 29/01/15
Loeb Classical Library http://0-www.loebclassics.com.pugwash.lib.warwick.ac.uk/authors accessed 04/02/15
 Pausanias, Description of Greece I.34
 I Orop. 292
 IG VII 3499
 Apollodorus, The Library of Greek Mythology III.6
 Ovid, Metamorphoses IX.407
 fig.127 in R.Osborne 1998
 IG II3 1 445
 IG I3.1476 Petrakos "'Ιερον του 'Αμφιαραον" no.15
 Rhodes and Osborne 27
 IG VII 4253
 IG VII 4254
 IG VII 4252
 IG VII 235
 Thucydides II.23
 Thucydides III.91
 Thucydides IV.91
 Thucydides VII.60
 Isocrates 14.20
 Herodotus VIII.134
 Strabo 9.2.10
North West Attica -
38° 17' 28.2876" N, 23° 50' 46.77" E
Fig.1 Plan of the Sanctuary of Amphiaraus
Fig 2. Row of Dedicatory Bases at Amphiareion.
Fig.3 What remains of the theatre.
Fig. 4 The remains of the stoa at Amphiaraus
Fig.5 This is what remains of the larger of the temples depicted in fig.1