Complex sentence: On his way to Demotika, Kantakouzenos had seized a number of fortresses in Thrace and although another siege of Peritheorion failed he followed up with a successful campaign that took Komotini and other fortresses in the Rhodope area.
Split into simpler sentences: On his way to Demotika, Kantakouzenos had seized a number of fortresses in Thrace, although another siege of Peritheorion failed. He followed up with a successful campaign that took Komotini and other fortresses in the Rhodope area.