It gave Kafka nightmares but everyone else seems to like it just fine.
Prague Castle is the symbol of all things Czech and it's not easy to forget this as you look up at it sitting high above Mala Strana. It's unique appearance is due in part to the fact that every major period of architechture throughout human history is represented somewhere in its walls. The castle itself was founded in the 9th century by Prince Borivoj and has enjoyed major periods of reconstruction and addition, mainly under Charles IV and later Renaissance additions under Rudolph II, when the Castle enjoyed its cultural Golden Age.
It is dominated visually by St. Vitus's Cathedral and also houses the Royal Palace (Kralovsky Palac) and the tiny Golden Lane (Zlata Ulicka), where Kafka once lived with his sister in 1916-7.
You will want to plan at least a day to wander around, among, and through the rich architectural and artistic rewards that Prague Castle has to offer you.
There are many ways to this, the most common of which is to take the #22 or 23 trams up to the ""Prague Castle"" stop. Cross the street and walk to the castle over the Deer Moat, arriving in the Second Courtyard. There you can either turn left towards St. Vitus or right, to the First Courtyard, where you can see a gorgeous view of the city.