One thing that's needed is something to break up that its flat walls, you need geometry to stick on walls and ceilings to break up endless corridors of flat walls, so buttresses and beams, enclaves, vaulted ceilings, etc etc, otherwise its just flat walls going on and on.
see here.
(Fixed link)
http://game-server-hosting.net/dantes-inferno/
Those walls are not flat.
Things of interest to draw the eye
Gothic example of 'greeble'
Here's an extreme example
See these old games - Quake 1
Quake 2
Completely random Quake 3 image
Just completely random googling of quake 3, flat walls broken up.
See that stuff poking out the walls. No flat boring walls here.
That's a VERY old game, it doesn't use flat walls, Ive mentioned this before at FPSC, fpsc users are obsessed with tiny insignificant objects, spoons, little tables, CD players, and they spam them into awful none existent architecture, get your layout up to scratch, don't concentrate on nik naks, get a decent layout, this is what having a grounding in old school level design gives you, box rooms connected with box corridors, filled with nik naks, does not make for good level design. We need to get fpsc into this decade, at the moment its level design is like classic doom, not even quake 1, lets break up those fpsc flat walls with some architecture design.
And for an idea for you.
How about making causeway sections, always good for a dungeon.
