I've always thought of making a SuperMario64 map, specialy the spiral mountain on the last screen shot. I'll give a 10 for design because I doubt anyone would take their time doing all those textures and stuff (even though they are pretty simple, which is fine...).
Gameplay is rather short but still quite fun. I hadn't expected a L4D map on this.
Looks are ok. The front part of the castle could be worked on a little more (a few displacements here and there, making the mote round, better lighting...).
My favourite part is the mountain. The textures are quite like the real stuff and the area is recognizable because of the floating island in the background. In the actual game, this area is brighter and fogless, but since this is L4D, it's alright.
Pros: Fun Looks, good flow
Cons: WAY TOO SHORT! Me and my buddies just took about 5 min to do it on expert mode.
Improvements: you could have the next part spawn you back in the main part of the castle just like in SM64 and do a different mission like the snow level, or the volcano level.
Notes: Many of us played the game and know our way around all the courses too well, adding obstacles and more challenges to the mix would help. the last level should be battle Tank Bowser at bowsers arena
Looks awesome, my only complain is that the front of the castle is just plain, but anyways, good job!, not only this map is good but also brings back a lot of good memories!
As for other maps, I don't know, you can do one of the snowy levels, or maybe a Mario Kart level.
It whould be cool if u made all the rooms in the casel,
(for exaple): the garden with all the ghosts in, and so on.
Like the Mushroom Massacre to garrysmod: http://www.garrysmod.org/downloads/?a=view&id=21003
I don't have L4D, but I would definitely want to play this map.
Looks great; however I would suggest matching your fog color to the skybox; the white fog looks weird against a darkish blue sky.
I assume you've ported at least some of the map geometry directly from the N64 right? Why don't you do another, like the dungeon/cave level?