Martyrium

[2] Martyria, mostly small, were very common after the early 4th century, when Constantine and his co-ruler, Licinius, became the first Roman emperors to declare religious tolerance for Christianity in the Roman Empire (Edict of Milan, 313 AD).

Martyria had no standard architectural plan, and are found in a wide variety of designs.

There was often a sunken floor, or part of it, to bring the faithful closer to the remains of the saint, and a small opening, the fenestella, going from the altar-stone to the grave itself.

Constantine the Great applied this style to the tomb of Jesus at the Anastasis in Jerusalem (c. 326–380s) and the Apostles' Church in Constantinople, while also erecting round mausolea for himself and his daughters.

[5] The same form was later adopted by early Islamic architecture, which employed it in the creation of a shrine known as the Dome of the Rock in Jerusalem, built much in the style of the Constantinian rotunda of the Church of the Holy Sepulchre, with which it was meant to create a "dialog of shrines", while standing at a prominent, isolated position – the Temple Mount.