In that case there was usually a triple apse. The design was complicated by a pre-apse space. The vaults were semicircular and built of bricks. Some churches had outhouse parts such as galleries (SS. Forty Martyrs Church and "St Peter and Pavel" in Tarnovo), chapels (Boyana Church), ossuaries and others. The Church "St Virgin Maria of Petrich" in the Asenova krepost has two stories and the lower one served as an ossuary. A peculiar type of Christian churches were those with a triconch plan. They are small, one-naved with or without a narthex. Their main peculiarity were the three conchas (apses) placed in the eastern, southern and northern walls of the naos. The small dome is laid directly on the walls. The conchal churches can be found mainly in the monasteries and are not so exquisite. Examples include the church in the "St Archangels" Monastery in Tran, "St Nicola Mrachki" in the Peshterski Monastery and others. The main feature of the Tarnovo School of Architecture is the rich decorative ornamentation in exterior of the edifices. The technique that was used was called mixed construction in which belts of stone and bricks alternated with each other. That method was a heritage from the Roman architecture (opus mixtum) but in that case it had lost its initial constructive application and was used mainly with artistic purpose. The wide plaster fugues also had decorative purpose. The builder chose different types of stone (limestone, travertine, marble, granite), and the bricks had varied shape and dimensions and were put in different positions forming decorative ornaments and monograms. A cell type method was sometimes used in which each stone block was completely surrounded by bricks. The bricks were painted in red in order to increase the contrast. The facades of the churches were segmented by deep niches (often with two steps) decorated with flying buttresses and archvaults. Unlike the churches in Constantinople the niches from the Tarnovo School of Architecture do not respond to the interior (pseudo constructive niches) and were used for purely decorative purpose. As an additional decoration rows of clay circles and four-leaves glazed in green, brown, yellow or orange were used. The origins of that type of decoration are in the Arab architecture. The Nessebar churches "St John Aliturgetos" and "Christ Pantocrator" have especially sumptuous decoration.