The fifth century rotunda near the Myrelaion church (Bodrum Camii) was the largest circular building in Constantinople. Later it was in ruins when its foundations were used as a cistern and a palace was constructed over it, perhaps by Romanos I Lakapenos who also built a church adjacent to it. Today, the former rotunda appears as a row of relatively narrow, low, and dark chambers which house a bazaar for leather goods.
Probably dating to the fifth century, the rotunda had an outer diameter of more than 40 meters, making it the second largest circular structure of the Roman-Byzantine period (only the Pantheon in Rome surpasses it). It can also be compared with other rotundas, for example the Rotunda of Galerius in Thessaloniki or the Red Basilica in Pergamon. It was situated on the south side of the main street from the city center towards the Golden Gate, not far from the Philadelphion. The chambers date from the middle Byzantine period, when they replaced the dome and served as a cistern as well as substructure for a palace which was built on top. The complex was referred to as the House of Krateros and later owned by Romanos Lekapenos, who turned it into a monastery after his accession to the throne in 920. The monastery also comprised a neighboring church, which became known as the Myrelaion and was afterwards converted into the Bodrum Camii. Excavations of a huge rotunda next to the started in the 1930s and were completed in the 1960s.
The cistern and palace were studied by K. Wulzinger a century ago, when the rotunda was not yet visible. The latter was only discovered in 1931, when T. Macridy and D. T. Rice excavated the substructure of the Bodrum Camii and uncovered a small outer section of the rotunda. This was then excavated in 1965/66 under the direction of R. Naumann. The findings include twenty-six pilaster capitals or capital fragments as well as figural floor mosaics now at the Istanbul Archaeological Museums. In addition, a fragment of a porphyry sculpture – the heel of the Portrait of the Tetrarch was found on site, proving that this sculpture group in Venice originally was located in Constantinople.
The rotunda lies at the core of a larger building complex. It had two opposite doors, a wider one on the north and a narrower one on the south. The northern portal was 3.70 m wide and opened onto a portico. The walls flanking the portal form an angle and indicate that the portico was bent with a radius of approximately 40 m. It may have surrounded a sigma-shaped forecourt and possibly a u-shaped extension up to the main street which passed by on the north. Other Constantinopolitan rotundas were similarly combined with sigma-shaped forecourts, e.g. the palace complex at the Hippodrome and the (later?) bath or baptistery in the Mangana quarter. In all cases the forecourt will have marked the main entrance. The narrower southern door was 2.85 m wide and opened onto a semicircular flight of stairs. They led to a lower level, as the ground slopes down toward the Sea of Marmara. Here the rotunda was flanked by two separate porticos on the west and the east. Roughly 40 m to the south stood a second, smaller centrally-planned building almost on the same axis as the rotunda. This second building was excavated by Rice on the same occasion when he studied the Bodrum Camii, but has since vanished. It seems to have been contemporary with the rotunda. The masonry of alternating layers of blocks of limestone and brick is similar, and the same mullions have been found 11 m to the south of the rotunda as well as in the second building. Their narrow-pointed acanthus resembles that on a console from the rotunda itself. 
The original function and identity of the rotunda are disputed. One of the possibilities, taking into account the probable fifth-century date of the complex, is that it was the palace of the Theodosian princess Arcadia. Moreover, Naumann suggested that the sigma portico of the rotunda could be identified as the Amastrianon which served as a market and place for public executions in the Medieval period. According to Byzantine texts, the Amastrianon was an open space limited by a semicircular portico and adjacent to the Mese approximately in the location of the Myrelaion rotunda and it was defined by a semicircular portico and a straight wall which contained a gate. The Patria (10th century) tell us other interesting details: For example, there were several reliefs and statues in the Amastrianon court, such as Zeus driving a chariot , turtles, a personification of the Lycus River and a relief of a man from Amastris with his servant. In the 8th century the rotunda behind the portico must have been in ruins (confirmed by the twentieth-century excavations) and it is quite possible that the sigma court may have become a market place by that time.
The ruined rotunda was re-used when a new palatian building was erected directly on the top of it in the Early Middle Ages. Before the construction of the new palace started, the walls of the rotunda had been consolidated and both doors walled-up. The interior of what had left of the late antique structure was converted into a cistern whose vaulting, supported by re-used columns, created a leveled platform on which the medieval palace was built. The palace was considerably smaller than the rotunda and occupied only the eastern part of the platform. It consisted of a rectangular hall (21.75 x 7.80 m) with two corner tower-like structures connected by a portico. Thus, the building resembled a Roman corridor villa. The building was identified as the palace which belonged to the emperor Romanos I Lakapenos (920-944) and was converted by him to a nunnery, as it is known from the sources, situated near the Myrelaion church (also work of Romanos) which fortunately survived, converted into a mosque (Bodrum Camii). Nevertheless, it is not certain whether Romanos built this palace. In this respect it has been suggested that the palace may be the same as the House of Krateros adjacent to the semicircular Amastrianon court (either the general Theodore Krateros serving under the emperor Theophilos (first half of the 9th century) or Andreas Krateros who was a domestikos in the reigns of Basil I and Leo VI (second half of the 9th century).

Also see Myrelaion Church