Nicosia's sights are concentrated in and around the Old City, surrounded by a picturesque star-shaped city wall whose moat has been converted into a pleasant park. Wandering around the Old City is an interesting experience in itself, although some buildings esp. those near the Green Line are derelict and crumbling. Note that many sights in the Old City close early, so try to get an early start - also a good idea for beating the heat in the summer.
