Castle on the Coast (UP)

After meeting the very obvious soon to be villains, you are instructed to gather the keystones to open the tower.

Leaving this room, go to the left most side of the room and solve the pressure plate puzzle to begin your trek to the Forest level.

On your way there, you will encounter a room with a sandy looking floor. There is a hidden ball behind the pillar on your right once you enter the room. Jump on it to pop the achievement.