On a budget, Prague Castle still earns its price.
Grounds free; circuit ~450 CZK
Less a single castle than a walled town of palaces, churches and gardens; the scale and the views back over the city are the payoff. The catch: the 'free to enter the grounds' line is misleading — the buildings worth seeing need a paid circuit ticket, the security line can swallow 30+ minutes in season, and it's huge enough to exhaust you. Half a day minimum if you want more than a walk-through.