During the early Middle Ages, the people of Paris poached entire sections of the sturdier Roman walls to use for their own buildings. You can still find roman stones in the cellars of the medieval half-timbered buildings found mostly in the 4th arrondissement but also in the 1st, 2nd, 3rd, and 6th.
Interiors have low pier and beam ceilings, walls are half-timber or exposed stone, and the layout tends to have small and irregular sized rooms. These Pain de Bois buildings are structured around a staircase with a wooden or stone load-bearing axis at the center and a cellar of vaulted caves, both of which make installing an elevator in these buildings almost impossible.
Religious buildings also survive, beginning with the Romanesque style (round arched windows) and then evolving into Gothic style (pointed arches, cross vaults and flying buttresses).