He-Man and the Masters of the Universe (2002) 1x14

With the peace talks between the Speleans and the Caligars having broken down, King Randor, Prince Adam, and Man-At-Arms journey to Subternia to help them reach a settlement. At the same time, the Evil Warriors decide to attack Castle Grayskull form below as all of their other methods have thus far failed. It is not long before the two groups meet in the depths of Eternia...