A Switch Palace is a short level where a very large P-Switch of a certain color can be found, depending on the color of the Palace visited. Switch Palaces are only found in A Koopa's Revenge II, but if you were looking for the small blue button labelled "P-Switch" from A Koopa's Revenge, then click here.
In certain levels of the game, secret goals lead to one of the three Switch Palaces in A Koopa's Revenge II. Pressing the Switch at the end of the palace will activate blocks of a certain color based on the same color of the palace and Switch itself. It will then serve as a solid block, and, depending on placement, serve as a helpful platform to make levels easier.