1) How do you come up with an initial concept?
For me, I seem to be able to plan my levels best when I play many popular levels, try to see what makes them so good and/or (re)playable, and then imagine a basic layout that incorporates these things. Basically, I just look at other levels and do the rest in my head, which probably isn't very helpful. I seem to subconsciously try to do the thing that Issac pointed out in most levels.
Sometimes I entertain myself with the questions "How would this work out in the real world? What would it be? What would it do?" etc. I don't know exactly why, but this seems to help me out most of the time. But when I speedmap (i.e. half an hour to an hour to make a complete level), I don't do this much.
2) How do you decide placement of enemies, and key objectives (reactor, exit, hostages, keys etc)
Whenever possible, I try to actually build the level with such functions in mind. I usually will build areas where I intend to put robots, and then put robots in them later on. On the other hand, I will also throw in surprises, so you're not gonna have to deal with relentless closet monsters. The reactor room is always a planned area, and usually very far from the starting position so as to allow the player to have to fight through the level to it. Again, mixing things up is cool, too. One level concept I have is simply blowing up the reactor at the start and then finding the keys afterward. Another is to blow up a boss robot to obtain a key and then have to go through untraversed hell to get to the exit.
So this kind of stuff comes right after the initial concept, where I decide how the level would best accommodate these things, and then make sure the level does so.
3) How long does it take to develop a single level (roughly)
Well I tried speedmapping a few times and managed a playable level in a single hour. I have a short attention span though, so when I try to make a quality level, it can take a couple of days. What helps me is to listen to atmospheric music, such as Future Sound of London
. When I do this, I can make a good multiplayer level in two to three hours, completely textured and full of all weapon pickups. A single player level can take a couple of days even when I do try things like listening to music while levelbuilding. To this day though I have only managed two single player levels, but one of them does not count and the other is going to be part of a pack.
Since I do not do this for a living, I sometimes find massive breaks between the times I develop levels, following wherever my interests meander to. This includes making Atomic Moth Invasion levels in Jane's Fighters Anthology
I do not only play the Descent games, but I will answer these anyway:
1) Which do you prefer, equipment and boxes everywhere (like D3) or stark caves/buildings (like D1 and D2)
I don't know what kind of game this is really supposed to be. In single player levels, clutter is fine. But in multiplayer levels I don't want that stuff getting in my way. I learned this the hard way by playing some of my more detailed levels (such as Nocturne). It wasn't (as) fun because the pretty parts of the level just
got in the way. So my newer levels (Boxwerk, upcoming level called Seven) do away with this kind of nonsense, for the most part.
However, in these days you'd better not just make all your levels a bunch of boxes and expect people to like it. It's fine in Descent, but not in newer games.
2) Do you prefer maze type levels, or simple "blast in, kill as much as possible, escape" type?
A mix of both is great in single player. You should have much less elements of maze-levels in multiplayer though. Corridors are great in both, but multiplayer levels should be straightforward in the "How Can I Get To Killing As Many Players As Possible As Soon As Possible" department. I do not have as much experience with single player levels but you can have a lot more maze elements to these. That's what makes a lot of Descent single player so great. \o/
3) Is there a place for puzzles or challenges in a level (like the pillars thing in D3) (as part of this, are objects like "teleporters" a good game-play item)
Lots of old school video games have such puzzles. I didn't care for the pillars thing though. I also dislike when puzzles seem to be.. out of place, redundant, needless, and excessively tedious and/or difficult. For example, there's a level in Marathon which requires you to set the height of a bunch of platforms *JUST RIGHT* so that you can get to the next part of the level, and even the most seasoned Vidmaster hated it. So in the free release version of the game, the platforms automatically stop to the right height. Puzzles shouldn't break the flow of gameplay by any means.
I am not against the use of teleporters. Halo has teleporters and they're used
all the time.
4) What makes some levels more-replayable than others?
Being memorable and/or fun. That's it. Replayable multiplayer levels are generally the ones which flow well and have something for every or nearly every playing style. This is why Blood Gulch in Halo is so popular: no matter how you like to play the game, you can do so in your own way and not only do great things for your team, but also have fun doing it. It accommodates everyone. At the same time it is straightforward enough that it does not take a genius to figure out how the level works. As for memorable single player levels.. they're just the ones that are fun.