Piston maze rails: one of the rails are set to stop the cart (the first one after you suffocate through the wall)
Entrance to piston maze: structure is visible (see image)
Fourth area (large room): Err... can't see a switch anywhere for a door. Why not just knock the door down, like the zombies do in hard mode? <object class="emojione" data="
https://resources.enjin.com/1489581540/themes/core/images/emojione/svg/1f600.svg?0" type="image/svg+xml" standby=":d">:d</object>
Past the impossible door: More impossible doors. How do you open these?
Other than these, the levels seems quite bug-free.