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From Wikipedia:
The Convent of the Order of Christ in Tomar was originally a Templar stronghold built in the 12th century. After the Order of the Knights Templar was dissolved in the 14th century, the Portuguese branch of the order was turned into the Knights of the Order of Christ, which supported Portugal's maritime discoveries of the 15th century. The castle Romanesque round church (seen at picture) was built in the 2nd half of the 12th century. From the outside, the church is a 16-side polygonal structure, with strong buttresses, round windows and a bell-tower. Inside, the round church has a central, octagonal structure, connected by arches to a surrounding gallery. The general shape of the church is modelled after similar round structures in Jerusalem: the Mosque of Omar and the Church of the Holy Sepulchre.