Well, its perfectly doable. Nyzul was a tower-like system at FFXI.

I do enjoy tower systems too. Hopefully the new dungeons will have similar system...i love having the boss at dungeon end and beating it unlocking a harder new part/version of that dungeon.

Dragonica (F2P MMO) has an excellent tower system...pretty much every dungeon there uses this system. You can have from 1~4 floors quick towers to up to 50 floors towers.

I do think it would bring a very interesting challenge to players since they can just choose dificulty based on floor. Thumbs up for the idea! ^^