A martyrium is a structure built at "a site which bears witness to the Christian faith, either by referring to an event in Christ's life or Passion, or by sheltering the grave of a martyr". Martyria, mostly small, were very common after the early 4th century, when Constantine became the first emperor to make the Nicene Creed the dominant religion of the Roman Empire. 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.
Gradually they turned into churches, or annexes to them. Later churches began to bring the relics of saints to the church, rather than placing the church over the grave; the first "translation" of relics was in Antioch in 354, when the remains of Saint Babylas (which were in a sarcophagus) were moved to a new church.
Martyria that remain in something like their original form include the following:
- the 4th century core of the much expanded St. Gereon's Basilica, Cologne
- A building with three apses over the Catacomb of Callixtus in Rome
- Santo Stefano Rotondo in Rome, late 5th century
- Basilica of San Lorenzo, Milan, perhaps 4th century, although the oldest part of the church now evident is an adjoining Imperial mausoleum of the 4th century (compare Santa Costanza in Rome).
- Church of the Holy Sepulchre in Jerusalem, on the most important Christian site of all, founded by Constantine.
- Church of Saint Simeon Stylites, Syria, 5th century, also very large, and now in ruins.
- Dome of the Rock in Jerusalem, erected in the style of the Church of the Holy Sepulchre by Abd al-Malik of the Umayyad Dynasty.
- Krautheimer, Richard. Early Christian and Byzantine Architecture. Yale University Press, 1986. Fourth edition, with Slobodan Ćurčić. p.518. ISBN 978-0-300-05294-7
- Syndicus, 73-74
- Syndicus, 73-89
- Syndicus, 73-87
- Eduard Syndicus; Early Christian Art, Burns & Oates, London, 1962