The Castillo de Santa Barbara is like a huge historic fort located right next to the amazingly…read morebeautiful ocean. The entrance to the top of the Castillo though is quite interesting. You have to get in a lift, then walk through tunnels, dungeons and bridges. When you get up there, you will see pretty well-maintained Castillo. It's pretty touristy, but the view overlooking the whole city from the top is quite amazing.
Definitely a must-stop when you are in Alicante.