The massive, fortress-like Gothic cathedral dwarfing the rest of town is an unmissable reminder of Albi's violent religious past. The town was at the heart of the so-called Albigensian heresy of the 12th and 13th centuries and the bloody crusade that crushed it. Almost all of central Albi, including the cathedral, is built from bricks of reddish clay, dug from the River Tarn that meanders through the town.