And how exactly do the monsters get in there?

An explanation of how this is supposed to work would be great - I can't imagine this working.
And what's with the TNT there, if there's a monster on it it explodes or what - is pretty stupid since you can rebuild everything then.

And how to expand that (as stated in the description)? More doors???

I can only imagine that the monsters spawn above this apparatus, fall down here, the doors close and either burn ( lava ) or drown ( water ).
But I find the effort much too big for that.
Why don't you let the monsters take fall damage and use all the iron for funnels to collect the drops automatically????

DISLIKE

@thomy19 you're totally right about that, but monsters don't jump depths greater than 2 blocks....

@Luckyluke : What is your opinion referring to?

Quote from @thomy19 : "Why don't you let the monsters take fall damage and use all the iron for funnels to collect the drops automatically????"@thomy19

@Luckyluke : There are monster traps that work with water. The monsters fall into a moat, are flushed towards the "hole" and then fall down. On impact, they then die.

That makes this whole apparatus of this idea futile

@thomy19 you must first create, but is possible. Have you just something incomprehensible formulated.

@Luckyluke : Already have two of those monster traps sitting around. One automatic and one XP farm

@thomy19 o k

All known, wooden doors would have done (except in hardcore mode because of zombies).