Biggest castle in Europe! Beautiful views of Prague from here.

Main attraction of Prague very very busy in the summer, you and your bag will be checked by the police before entering. It is a big building and very nice. Though it's not cheap to get tickets for the different areas to go in and you have to pay extra for taking photos. Note that you can enter the main hall of the cathedral for free, only pay for a ticket if you want to see the whole cathedral close by.

Also checkout Vyšehrad which is much more quiet and visited by locales and has an arguably better view then the Prague castle. And much cheaper food/ drinks. Suggested by the honest guide of Prague on YouTube.

Biggest castle in Europe! Beautiful views of Prague from here. History is so well preserved. Please note that this is a steep climb through the city's streets as well as up the 208 steps so if you have any disabilities you might want to consider a tour bus.