Černá věž (The Black Tower)
The word 'castle' to me conjures up an image of a heavily fortified stone building complete with battlements and towers.
There is very little of that about Prague Castle (Pražský hrad). Only on the northern side and at the eastern end are there significant remains of the old fortifications.
Although started in the 9th century, the castle has undergone many phases of building and rebuilding, particularly following the disastrous fire of 1541 which destroyed large parts of the castle.
For information on ticket prices, opening hours, etc. please see the Official Site, detailed below.
Only mentioned in passing on the official site are the Audio Guides. Whilst I can strongly recommend these in themselves, they are very interesting, they also give you priority entry to St Vitus Cathedral. On a freezing cold day in March, they are pretty much an essential.