In Prague's Hradčany neighborhood, Prague Castle (Prazsky hrad), once the home of Bohemia's kings, is today the official residence of the Czech Republic's President and one of the city's most visited tourist attractions. Originally built as a walled fortress around 970 AD, the castle has changed dramatically over the years and contains examples of most of the leading architectural styles of the last millennium. Within the castle walls are a number of Prague's most popular tourist sites, including St. Vitus Cathedral, St. George's Basilica, the Powder Tower, the Old Royal Palace, and the Golden Lane. The largest ancient castle in the world, this vast complex requires considerable time to tour, but it's time well spent (particularly rewarding are the excellent views over the Vltava River with the old town and its countless spires in the background). Highlights include the Old Royal Palace's main hall, the Vladislav Hall, so large it could be used for jousting tournaments, and staircases wide enough to allow mounted knights to use them. Be sure to also spend time in the Royal Garden, dating back to 1534 and home to a number of superb buildings including the Ball Game Pavilion, the Royal Summer House with its Singing Fountain, and the Lion's Court. (English language guided tours are available.)
At night the lights from Prague Castle glow in a range of hues. Basing yourself in the vicinity is a good idea so you can experience the city highlights by day and night. For some accommodation ideas in the area see our recommended luxury, mid-range, and budget hotels near Prague Castle.