I usually place the models first. Don't just spam rooms, make each room feel as if it has a purpose. It's better to spend more time on a few rooms than spending time on spamming stuff. Avoid long corridors! :>
After that I create the lighting. Remember to have stronger lights where you place items and stuff important to the player. I don't like boxlights, work mostly with pointlights. Remember to not light up the whole room completely, have some dark corners; create contrast.
After that it could be nice to place some billboards if required, as well as particle effects. Lastly place decals on walls and floors (don't go for the usual blood everywhere!). You could take some decals from here:
http://cgtextures.com/ ----> Click "Decals".
Decals are a way to avoid repetition and create a story with your environment (I usually don't bother with these :> but I model stuff so I can adapt textures etc.).
Try creating rooms with interesting shapes, use welders and don't mix models which doesn't fit together. I'd recommend creating something other than the mansionbase/castlebase --> seems like the most used assets.
Most important, Have fun!
Quality > Quantity --> One map which has a lot of time spent on it is something I value more than 3 generic castlebase maps.
You could try making an interesting sewer environment. Browse the developement assets for custom models.