
Dragon nesting chambers
I think Fubar Obfusco brought this one up before; in any case, I'm going to bring it up again. These chambers would be small, roughly oval caverns, not necessarily all identical in shape and size, bored into the sides of wide corridors. The corridors going past them must be about 30 feet (3 dots) wide, for the sake of gaming feel and the laws of physics. (Dragons are big, even if they take up little space on the screen.)
Each chamber would have treasure stashed in it, and a bedding area (a ring or circle of rubble, but rubble that you can walk through), perhaps with 1 or more eggs, at the center. A dragon might or might not be in the chamber; if it is, it will probably be in the bedding area, and may be asleep. Note that attempting to steal a sleeping dragons booty may, as Bilbo's story shows, prove to be very dangerous, even for the experienced thief.
(Note: the more powerful (and thus larger) the dragon, the bigger the chamber.)
(Note 2: if possible, it should be coded for only Mature and older dragons to have eggs in their chambers, for "realism" purposes.)
Armouries/Weapons Storehouses
These would have racks of weapons, ammo, and armour, some (generally the ammo) powerfully magical, most standard. Armouries also tend to have at least a few orcs in them...
(Note: archer-type opponents (discussed previously) may restock their arrow supply here, if item-using monsters are implemented. Also, there may be wands, staves, and even a few rods in an armoury, making ones at lower levels worth searching.)
(Note 2: chests would be used heavily here. It is suggested that the "close" command be modified for use with chests and boxes...)
Supply Storehouses
Full of (what else?) rations and supplies. The food is generally edible, but usually for orcs, and therefore of rather poor quality (I'll just say that orcs have strong stomachs and few tastebuds). Supplies will probably include potions and such, perhaps with some medical potions (though, again because of the vast numbers of orcs, that is unlikely). Mushrooms of all kinds may also be found here - the orcs are sure to have their uses for them. Oh yes, and I almost forgot about slime molds... and potions of slime mold juice.
Farms
Big pits full of rubble, trash, and rock-eating fungus, and full of such atrocities as can eat that sort of stuff - namely, rock lizards and that sort of thing. There may also be giant ants, and the occasional beatle as well.
Wonder why orc food is so disgusting?
Graveyards (for lack of a better name)
No, these aren't undead pits - but they are, literally, pits. They're large pits with corpses in rows and mounds, and Sauron's necromancers and summoning specialists raising up lesser undead and minor demons of all sorts. Ah yes, it's so nice to know that you get a second chance at - er - unlife...

Barracks
Come on - orcs need to live and sleep somewhere! Barracks are more like prison-type vaults than anything else, with cells containing orcs, both sound asleep and wide awake. Going in here might be a bad idea...
Sorry, but that's all I can come up with for now. What do ya think? Any other ideas?