One of the most iconic Czech castles. Built in 1348 by the Czech King (and since 1355 Roman Emperor) Charles IV., it used to be the treasury and summer residence of Czech Kings. Today it is one of the most frequently visited Czech places outside Prague. It is quite close (about 30 km South-West); visitors of Prague can easily find a guided trip from Prague, or just use a train (from the station Praha-Smíchov) or a rented car (there is a big parking site under the castle near the river Berounka; cars are not allowed in the town itself). It is a 15-20 minutes walk from there but the street is quite steep, so if you have health issues, you need to seek a special tour with transportation all the way up to the castle.
The location itself is very nice and there are plenty other places to visit nearby like tripple lakes (former quarries) called Velká Amerika, Mexico and Malá Amerika (Big America, Mexico and Little America) or the stalactite caves called Koněpruské jeskyně, or a nice historical town Svatý Jan pod Skalou. This area near Berounka River is called Český kras (Bohemian Karst).