× there is one trapdoor that is not in the line of the trapdoors.

× It is near the upper left area.

×
First Delver! (I was the first non-tester/dev
to conquer TCB.)
d/dy

×A - first find the hidden trapdoor. To do so, just walk around killing the tar next to the passageways as well as the tar on your path. It's quite likely you'll block yourself from winning this way, but once you find the hidden trapdoor (there's only one), it's time to:

B - Restart, solve the level normally, except when you get to the secret trapdoor, in which case just uncover it and step on it before proceeding.

Zex already hinted as to where the trapdoor is, so just start from the corner he mentions and it won't take you long to find it.