Complex sentence: The difference is that Sant Climent de Taüll has three naves and three apses, and a bell tower, whereas early Christian architecture has side aisles, transepts, a narthex and an atrium.
Split into simpler sentences: The difference is that Sant Climent de Taüll has three naves and three apses, and a bell tower. Whereas early Christian architecture has side isles, transepts, a narthex and an atrium.