Vyšehrad with Saint Peter and Paul basilica.
Vyšehrad was heavily destroyed during history. The remaining things are few buildings and walls. Especially in the spring and summer a small walk around the walls is so refreshing, relaxing and you can enjoy the historic part with amazing view on Prague and Vltava.
There is a legend about Horymír, who was senteced to death. The army caught him on cliff but his brave horse, Šemík, decided to jump down from the very high reef. The horse died during the jump, but Horymír survived. The prince was so amazed that he gave him a grace.