Complex sentence: Cofitachequi was a large town in addition to being a ceremonial center as de Soto took over about one-half the houses in the town for his 600 men (and probably several hundred Indian retainers and slaves).
Split into simpler sentences: Cofitachequi was a large town in addition to being a ceremonial center. The size of Cofitachequi is indicated by the fact that de Soto during his occupation took over about one-half the houses in the town to house his 600 men (and, in addition, to house probably several hundred Indian retainers and slaves).