You could easily spend all day here...I didn't have that much time to spare as I was only in Prague for a couple of days. Get there before midday for the changing of the guard and make sure you're at the front as it does get busy. It last about 15 mins and remember you'll be standing in the midday sun!
There's a security bag check before you can enter the castle grounds, but they're very quick. Just be prepared for it. To enter the grounds is free, but if you want to enter the castle etc there is a charge. Like I said I didn't have the time, but I'll be back to do this lovely place justice.
If you’re in Prague, you absolutely have to visit the castle.
It’s not actually a castle per se but a load of historical buildings, cathedral, little houses, shops and gardens within the castle grounds. You can walk around the compound for free but I would highly recommend paying for one of the circuits so you can actually go in the buildings.
If you’re not fussed by art, you can save some pennies and go for circuit B which gets you in to everything minus a couple of art exhibitions- and cost us less than £10 ! You can easily spend several hours to most of the day there.