Although the building itself is of an immense size, it was only one part of a much larger sacred complex, surrounded by high walls, that dwarfed even the colossal Temple of Jupiter in Baalbek.

A series of tunnels and chambers lies under the main temple, connecting it with the side rotundas and giving private access to different areas of the complex.

Various drains, water channels and basins are located in, around and under the main temple and may have been used for symbolic reenactments of the flooding of the Nile.

Its use of red brick on a massive scale, unique in Asia Minor but relatively common in Italy at the time, indicates that the architect was not local.

The immense size and lavish construction of the complex points to an extremely wealthy patron who sent a Roman architect and brick masons to Pergamon to build the temple.

He is known to have been an enthusiastic sponsor of the Egyptian gods; he built temples of Isis and Serapis at various places in the Roman world, including at his own villa in Tivoli.

It was not restored, but was redeveloped in the 5th century AD as a Christian basilica, built inside the shell of the destroyed temple.

[2] The church was probably destroyed by the forces of the Arab general Maslamah ibn Abd al-Malik, who besieged and looted the city in 716–717 during his unsuccessful bid to conquer Constantinople.

Most of the temenos was destroyed and built over long ago, but substantial fragments of the walls remain standing to a height of 13 m today.

[5] The temenos was built on top of the River Selinus, presumably because the person who commissioned the complex wished it to be located in the city centre rather than in an outlying district.

As the city was already substantially built up, the river bed offered an otherwise unused location for the temple complex and reduced the number of properties that would have to be demolished to make way for it.

The river was channelled into two tunnels passing diagonally for a distance of about 150 metres (490 ft), northwest to southeast, under the temenos and temple.

The building was originally a vast hall, rather than a basilica, covered by a wooden roof that had no interior support or colonnade.

In front of the door stood an iron grating, which presumably had an opening in it to permit access to the interior of the temple.

[8] Near the centre of the hall is a shallow basin, 22 centimetres (8.7 in) deep by 5.2 metres (17 ft) long, in which three rectangular tubs stood, placed parallel to each other.

The one on the south side, which is part of the Red Basilica archaeological site and is open to visitors, was re-used and modified in the Ottoman period, and in the 19th century became the machinery room for an olive oil factory.

They appear to have been generally unadorned, but some pieces of coloured stucco are reported to be visible in one area – perhaps the remainder of some kind of decoration.

An inscription referring to the temple mentions "Serapis, Isis, Harpocrates, Osiris, Apis, Helios on a horse ... Ares and the Dioskouroi".

The purpose of its high walls was to prevent outsiders from witnessing ceremonies held within the temenos and temple precinct, thus preserving the mystery of the rituals.

Initiates may have been taken through the underground passages to the cultic area, where they would be presented to the worshippers filling the western end of the temple.

Something of this nature is hinted at by Apuleius in Metamorphoses: "There in the middle of this sacred temple before the image of the goddess I was made to stand on a wooden pulpit.