Thwomp is an enemy that appears in Whomp's Fortress and Tick Tock Clock. Thwomps are large, blue, block-like enemies who "attack" by moving downwards, crushing Mario if he is beneath them. Mario can climb on them and is intended to use them to reach higher areas in some cases. A very similar enemy called Grindel appears in Shifting Sand Land.
When Thwomp’s rising action begins, Thwomp begins to rise at a rate of 10 units/frame for 40 frames and then rises an additional 5 units and stops. Thwomp remains in the air for a variable period by calling for a random number between 0 and 1 which is then multiplied by 30 and added to 10. This is then used as the number of frames before Thwomp will move back down again. When this happens, Thwomp accelerates downward at a rate of 4 units/frame per frame. When Thwomp hits the ground, its Y-velocity is set to 0 and, if Mario is within 1500 units of Thwomp, the screen shakes and Thwomp makes a sound. After 10 frames, another random number from 0 to 1 is rolled and this number is multiplied by 10 and added to 20 to get the number of frames that Thwomp is on the ground before it begins to move up again.