What was the focal point of the building in most central plan churches

Central Dome

Explanation:

Most churches are constructed and planned to ressemble a giant cross and where the lines of the cross intersect they almost always put a dome above it, most catholic churches from old times are planned and build like that, with two altars in the sides of the cross dedicated to diverse saints and the main altar in the upper side of the cross, with a giant dome in the middle.