It is plain, geometric, flat, straight lines - just boxy. It really looks like a fortification, it would perform its intended function well, it just isn't the prettiest. A couple things I liked were the moss growing on the side of the walls, the views of the city below, the view of the sea (except where you see an industrial looking port) and the multitude of cannons. Check out the pictures below to see what I mean.
Hours: Oct-Mar: 9am-7pm Mon-Sun
Apr-Sept: 9am-9pm Mon-Sun
Time to self-tour: 30 min-1 hour