Some ideas...

Since I made a Stone Fortress and a Marble Fortress, I am thinking of throwing in a wood, metal, and magma fortress somewhere in the wad as well. Maybe Magma Fortress will be my 18th map since map17, which I'm getting near to finish, seems to be mostly a magmatic level.
I may need some ideas for a 2nd secret level too. I attempted to make it like a part 2 of The Absolution, but couldn't get the level just right since I wanted it to have the 666 tag effect that map32 does not have, unless I do a little MAPINFO lump editing.
I'm thinking of putting in a couple of effects such as a see-through door, which I almost successfully done in Marble Fortress, but couldn't get the ceiling flat to work right. The multiple-way bridges seem beyond my ability without really FUBAR-ing the level. Even when I look at the tutorials here, it doesn't really help me much.
Any ideas or suggestions either about the fortress levels, a 2nd secret level, special effects, etc.?

1. Sure, why not a magma fortress? I would make it more of a "cave" then a "fortress" though.

2. I have a tendency to make my last secret level REALLY hard to find, but to make it really silly or unique. Custom textures could come into use, I suppose.

3. If you're making the levels 4 a source port, load up on the special effects. If not, they'll still add to the game if you can do them right.

I have a bridge tutorial at http://www.geocities.com/jayextee

It's written a little easier to understand.

As for your secret level, just make something memorable. Something that will stick in a person's mind forever.

Mind, that's easier said than done... ;)