We bought the A Tour package, which included access to most of the main attractions to the castle. If you want to see everything, recommend either starting first thing in the morning or giving yourself two days so you don’t feel like you have to rush. We started late morning on the first day and came back early the next day to finish up in time for lunch.
The main attraction is St Vitus Church (# VIII on the castle map); it is impressive on its own. If you only see one thing at the castle, that’d be the one to visit.
They do a changing of Guards at noon; if you’d like to see that the best spot is near the exit at the front of the castle.More