Complex sentence: The Mongols destroyed most former centers of administration, which were defended earth-and-timber walls; only well-fortified places, including Esztergom, Székesfehérvár and the Pannonhalma Archabbey, could resist their siege.
Split into simpler sentences: The Mongols destroyed most former centers of administration, which were defended earth-and-timber walls. Only well-fortified places, including Esztergom, Székesfehérvár and the Pannonhalma Archabbey, could resist their siege.