Prague castle is a symbol of Czech state development from 9th century till these days (more than millennium). First building here stood from 9th century, later, in 12th century, it was known as Romanesque style palace, and Czech famous king Charles IV rebuilt it to Gothic appearance in 14th century. King Vladislav from Jagellonian dynasty (nice to hear about Lithuanian blood here!) reconstructed it in 15th century. Gothic Vladislav Hall is known as largest medieval hall in Prague.
Prague castle territory consists of such buildings as St. Vitus Cathedral, Golden Lane, St. George Basilica, towers, palaces and other objects. It is largest medieval castle complex in whole Europe, covers 45 hectares and represents actually all historical architecture styles.
Now Prague castle is used as museum and both as presidential residency from 1918. Price of tickets to castle varies depending to objects included. For me it was a bit expensive, and it was disappointing that there are no writings in English inside (one of most popular tourism objects in Europe…), so some people buy extra audio guides.