Standing on the Place Saint-Pierre at the foot of the castle, the Église Saint-Pierre is one of Caen's most distinctive buildings. The church was begun in the 13th century in a straightforward Gothic style, but its architects kept building and decorating for another three centuries, resulting in a building that layers Gothic, Flamboyant Gothic, and Renaissance elements in a way that shouldn't work and somehow does.
The spire — one of the tallest in Normandy before the 1944 bombardment brought it down — was rebuilt in the 20th century. The interior is relatively plain, the exterior the main attraction. It's a good five-minute walk from the castle and easily combined with a walk through the old market area around the Place Saint-Sauveur.