They are all square, with door with voussoir, floor and vault. The 4th century BC Rhodes Footbridge rests on an early voussoir arch. The entrance features a semi-elliptical rusticated voussoir arch with two Ionic order pilasters. At the outlet an arch of shaped voussoir stones supports the retaining wall above. Until the topmost voussoir, the keystone, is positioned, the vault is not self-supporting. The outer boundary of a voussoir is referred to as an "extrados". A gabled dormer and round-arched window with stone voussoir projects above each bay window.