When placing a thwomp in the level you have to have the correct graphics chosen. These graphic locations can be changed by clicking on the blue mushroom and changing the sprite graphics header to castle. This can also be done by clicking the red mushroom and changing the graphics page to that of the thwomp. Unfortunately at this time I have no idea which graphics page it is or which field has to be changed.
In short change the sprite header graphics to the castle setting, and you will have the correct thwomp graphics.