Zásadka Castle, Castle ruin in Sychrov, Czech Republic.

Zásadka is the ruin of a Renaissance chateau on a spur on the left bank of the Jizera above its confluence with the Mohelka, at 260 m, in the northernmost tip of the Central Bohemian Region near Mnichovo Hradiště. The oldest part is a partly buried two-wing palace with barrel-vaulted cellars on the north side; a representational Renaissance palace later took the northwest and southwest sides against the older perimeter wall. A square tower at the southwest corner survives only as a terrain relic, and ditches protected the front.

A small knightly fortress, first documented in 1406 when Matěj Pomazánka ze Zlivi bought it, stood here before Zikmund Vančura z Řehnic rebuilt it as a Renaissance château in 1560. Václav Budovec z Budova made further changes after buying it in 1604, and the Waldstein family held it until 1945. Fire damaged the château in 1790; it still had a roof in 1820 but became a ruin in the second half of the 19th century.

The name Zásadka comes from the knights who built the original fortress; in the late 15th century it was called Nová Zásadka after a rebuild. Since 1967 the ruin has been listed as a cultural monument of the Czech Republic.

The site is accessible from outside and allows visitors to explore the ruin's exterior and enjoy the surrounding landscape. The best time to visit is during dry weather, as pathways can be uneven and caution is needed when walking around the structures.

The oldest surviving part is a partly buried two-winged palace with barrel-vaulted cellars on the north side; the showy Renaissance palace was later built against the older perimeter wall on the northwest and southwest sides. A square tower at the southwest corner survives only as a terrain relic.