Complex sentence: While about 1,500 of Sweden's 4,000 churches date to the Middle Ages, the only secular buildings that survive from that period are a few Burgher's houses in Stockholm and Visby and some ruins of castles, fortresses and fortifications.
Split into simpler sentences: While about 1,500 of Sweden's 4,000 churches date to the Middle Ages, very few secular buildings survive from this period. There are however a few Burgher's houses in Stockholm and Visby, some castles, fortresses, and fortifications.