"Open/Sandboxy" maps and monster placement

[Cynical]

When I look at Golachab critically, one of the things I feel that I kinda did poorly with was the overall flow of the layout- it interconnects, sure, but it's a bit Room-Corridor-Room, and its fights are frequently a bit too neatly segmented. I've started on a map that intentionally works against this tendency -- the idea was to create something not too far removed from "Vanguard map 02 with semi-slaughter monster counts" -- and I'm really not certain how to approach monster placement.

One big problem I've run into is that frequently, just running is too much easier than actually doing anything to fight, and I'm not certain what the best ways to combat this are. One early thought I had was "teleport in Pain Elementals, if the player decides not to fight those, the world turns into Lost Souls", which partially works, but infinity-tallness can make that crap really annoying even when someone plays "honestly" (enjoy your invisible walls while 10 Revenants close in on you!). Cage-traps are also an option (I've got one in now), and it seems that Vanguard Map 02 sometimes solves this with platforming, which means I probably should go ahead and reshape the top of those pillars in the central room slightly to mandate killing those Viles... but what other options are there? Anything obvious that I'm missing?

Another problem I'm struggling with is how to create challenge other than "fuck it, more Revenants on the ground", given how easy it is for the player to run away in this kind of map. My precious, precious, Mancubi, in particular, seem to lose all of their effectiveness in this kind of environment. Arachno-snipers and Vile-turrets seem to work well, and I'm hoping that the Caco-swarms I have planned for later in the level are able to take advantage of the insane mobility they'll have, but overall, I'm kinda at a loss on good ways to use monsters- and the problem is going to get worse as the map goes on, since without randomly cordoning the player into areas (which is pretty much the opposite of what I'm trying to do here), there's nothing stopping them from running back into already-cleared areas to take encounters at whatever pace they want, which in turn lets them pretty much pick everything apart piecemeal.

Here's a super-pre-alpha version in all of its "default-Startan-textured-just-starting-to-lay-down-sectors-monsters-weapons-and-health" glory, for anyone who wants to have a look: http://www.mediafire.com/download/jaw9j4n0r94dqkf/Codex.wad . Complevel 9. Right now, it's possible to get to places that you shouldn't via Vile jumps (so, uh, don't do that), and there's nothing really after the Yellow Key (including no place to use it, right now). There *is* an exit, but it's not accessible without Vile jumping, and you shouldn't be getting to it yet, anyways (it's in an area that when I'm further along won't be accessible until you've got the Yellow and Blue keys- although, right now, in my "mental plan", the end of the level is changed enough that I'll probably move it).

[Phml]

Making objectives matter can help. You can have the path/room to a particular switch or key tricky to get to without killing monsters, or even better, tricky to exit from (ideally as in, by the time you go in and out the way monsters have moved during that time make them much more of a threat).

Good Plutonia-style maps exemplify that, by turning every item into an objective. You can run around in multiple directions and skip monsters, but it's likely your choices will be influenced by ammo or health placement, otherwise you tend to die. Items become path nodes, guidance for both the player and mapper while allowing some deviation.

[yakfak]

perhaps approach the map not as a series of orchestrated traps and goals so much as a fortress to which the player decides how best to lay siege.