+1
I was wondering what kind of new terrains the dev team would come up with for the sequel.
Looks like we have a seriously awesome candidate here. From a game design perspective, an underground level may lend itself well to F.O.W. (Fog of War) tactics a la games like Advanced wars and the like.
Basically FOW means that in a given environment (underground in this case) a tower has limited visibility (and therefore limited attack range). Some towers (like the
fire towers from Earendel's post) could increase their range of visibility (and that of their surrounding towers) as the player upgrades them. What also would be nice, is that for creeps outside the visibility of a tower, you only see their glowing eyes as they move through the darkness of the caves. And as for pop culture references I could easily envision the wailing echoes of some wretched creature muttering "my preciousssssss...." coming from the depths of the caves.