Complex sentence: Relatively few details are known of its history during the medieval period, however other than the castle it's principal engine of growth would have been its regular market, and many of its early buildings would have been grouped around the market area.
Split into simpler sentences: Relatively few details are known of its history during the medieval period. However, other than the castle, it's principal engine of growth would have been its regular market, and many of its early buildings would have been grouped around the market area.