Yeah, that's another arguable choice I made... The spell you speak of makes disappear things, thus you can only use it once (unless...). And the last door needs you to use it, but if you are a bit too far, it's dead...
But, there's a way to pass this sad event! (well, in fact, two ways...)
1) I forgot to remove a debug key that opens the door you aim at...
2) There's a glitch that let you fly...
Thank you for the kind comment! Happy that you enjoyed playing my game!
I think I'll try to push the project further.