There are straightforword reasons why they look like fortresses...

Many of the mosques in Our Fair City, especially the older ones, look more like a fortress than a mosque.

In a place where Muslims were often the unpopular minority rulers, one needed a place of refuge in time of trouble.

In the older parts of the city, I can still find bits of wall and ancient gates, leftover from the times when the Muslim parts of town were walled off.