Karlštejn is an amazing fairy-tale-like castle situated just 33 km from Prague. It was constructed in the 14th century by the Holy Roman Emperor Charles IV who used it as a place for safe-keeping his Imperial Regalia and Czech crown jewels.
The castle is very easily accessible from Prague. There are comfortable trains departing every hour from the main train station and the ticket will cost you less than 60 CZK. After you have climbed the castle hill you will be rewarded by an amazing view of the surrounding natural landscape.
See the official website of the castle for more information: https://www.hradkarlstejn.cz/en