Walls of Rhodes
One of the great medieval monuments in the Mediterranean, the walls of Rhodes are wonderfully restored and illustrate the engineering capabilities as well as the financial and human resources available to the Knights of St. John. For 200 years the knights strengthened the walls by thickening them (by up to 40 feet in places) and curving them so as to deflect cannonballs. The moat between the inner and outer walls never contained water; it was a device to prevent invaders from constructing siege towers. You can get a sense of the enclosed city's massive scale by walking for free inside the moat; entrances can be found at the gates of St. Athanasius and Ambroise. It's particularly stunning early in the morning or in the evening, and makes a great spot for kids to run around and play games.