An older rival to the castle
According to legend, Vyšehrad ("upper castle") predates Prague Castle itself as the seat of the earliest Czech rulers. Whatever the exact history, it's been a fortress, a royal residence and a citadel across different centuries, and today it sits as a large green park on a rocky bluff above the Vltava — with a fraction of the visitors you'll find at the main castle.
What's there now
The Basilica of St Peter and St Paul, with its twin neo-Gothic spires, anchors the site. Next to it, Vyšehrad Cemetery is the resting place of some of the most significant figures in Czech culture, including composer Antonín Dvořák and Art Nouveau painter Alfons Mucha, laid out around the striking arcaded Slavín monument. The old fortress walls and casemates are worth a walk for their own sake, and the ramparts give some of the best unobstructed river views in the city.
Why it's worth the detour
Vyšehrad sits a little apart from the main tourist circuit, reachable in a few minutes on the metro (Line C) from the centre, and that distance is exactly its appeal — it's the Prague hilltop-fortress experience without the queues, ideal for a slower morning or a picnic with a view.