The Church of the Theotokos, the oldest in the complex, is the only church known with certainty to have been built in mainland Greece in the tenth century. This centralized parallelogram-shaped building is the oldest example of the cross-in-square type in the country; its plan closely follows that of Lips Monastery in Constantinople. The Church of the Theotokos adjoins a larger cathedral church, or Katholikon, tentatively dated to 1011-12. The Katholikon is the earliest extant domed-octagon church, with eight piers arranged around the perimeter of the naos (nave). The hemispherical dome (without a drum) rests upon four squinches which make a transition from the octagonal base under the dome to the square defined by the walls below.[2] The main cube of the church is surrounded by galleries and chapels on all four sides.
