Did castles stop the Mongols?

Were castles the reason the Mongols never invaded Germany?

Not by themselves. Mongol armies could contain and starve a fortress while mobile forces operated across the surrounding countryside.

In this explanation, castles alone do not account for the Mongols' failure to conquer Germany. An impregnable fortress did not have to be taken by assault if a containing force could prevent its garrison from operating freely. The main mounted army could then destroy or seize resources in the surrounding country while hunger worked on the defenders. Fortification remained valuable, but it functioned inside a larger contest of mobility, supply, and control rather than as an automatic shield against invasion.