The erroneously named Temple of Minerva Medica is, in fact, a ruined nymphaeum of Imperial Rome, built in the 4th century. Nymphaeum is a building devoted to the nymphs and often connected to the water supply. The decagonal structure in opus latericium is relatively well preserved, the full dome having collapsed only in 1828. It is surrounded on three sides with other chambers added at a later date. There is no mention of it in ancient literature or inscriptions.
The structure represents a transition in Roman secular architecture between the octagonal dining room of the Domus Aurea and the dome of the Pantheon, in particular, and the architecture of nearby Byzantine churches. The diameter of the hall is about 24 metres, and the height was 33 — important from the structural point of view, especially for the ribs in the dome. In the interior are nine niches, besides the entrance; and above these are ten corresponding round-arched windows. Both the interior and exterior walls were once covered with marble.
References:The Walls of Constantinople are a series of defensive stone walls that have surrounded and protected the city of Constantinople (today Istanbul) since its founding as the new capital of the Roman Empire by Constantine the Great. With numerous additions and modifications during their history, they were the last great fortification system of antiquity, and one of the most complex and elaborate systems ever built. They were also the largest and strongest fortification in both the ancient and medieval world.
Initially built by Constantine the Great, the walls surrounded the new city on all sides, protecting it against attack from both sea and land. As the city grew, the famous double line of the Theodosian Walls was built in the 5th century. Although the other sections of the walls were less elaborate, they were, when well-manned, almost impregnable for any medieval besieger.