Karlštejn

One of the most iconic Czech castles. Built in 1348 by the Czech King (and since 1355 also Roman Emperor) Charles IV., it used to be the treasury and summer residence of the Czech Kings. Today it is one of the most frequently visited Czech castles. It is quite close to Prague (about 30 km South-West); tourists 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 lot under the castle near the Berounka river; cars are not allowed in the town itself). It is a 15-20 minutes walk from there to the castle 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.

More info can be found on the castle's web site or on Wikipedia.

The location itself is very nice and there are plenty other places to visit nearby, like the tripple lakes (former limestone quarries) called Velká Amerika, Mexico and Malá Amerika (Big America, Mexico and Little America, the whole place sometimes regarded as Czech Grand Canyon) or the stalactite caves called Koněpruské jeskyně, or a nice little historical town Svatý Jan pod Skalou (with a near open air museum of mining called Solvayovy lomy, again mind a steep path), or another monumental royal castle called Křivoklát. This area near Berounka River is called Český kras (Bohemian Karst).