English: This huge key stone which was found in Çemberlitaş may belong to a big arch at the Forum of Constantine. The forum was built by Constantine I in the quarter of today's Çemberlitaş. It was oriented north-south, the forum was oval in its ground plan. Historical sources report that located within the forum was:

- a Senate House or basilica, a monumental fountain or nymphaeum, and silversmith and chandler shops.

The pavement was of marble. On the periphery it was ornamented by statuary and reliefs. An honourific column dedicated to Constantine I was erected in its centre. A place of worship devoted to the emperors, known as Hagios Constantinos, was constructed on the rectangular base of the monument. One of the official imperial ceremonies was held here. Basil I the Macedonian constructed the Church of St. Mary (Theotokos) on one of the porticoes.