Complex sentence: Meanwhile, he had assembled a considerable force, with which he attacked Syracuse; and having defeated Dionysius in a decisive action, made himself master of the whole city, except the island citadel, in which he kept the tyrant closely besieged.
Split into simpler sentences: Meanwhile, he had assembled a considerable force with which he attacked Syracuse. He managed to defeat Dionysius in a decisive action, making himself master of the whole city, except the island citadel, in which he kept the tyrant closely besieged.