Complex sentence: The walls of the citadel expanded over the Late Helladic III period, with LH IIIB being a time of competition between regions leading to "enlargement" of the cities.
Split into simpler sentences: The walls of the citadel expanded over the Late Helladic III period, the reason believed to be region competition. In the LH IIIB, time of competition between regions lead to "enlargement" of the cities.