Man, not much going on in the forum. Yawn. Digging through old threads.
I know, this is an extreme bump, but the thread contains good and broad knowledge relevant for all mappers, so bumping it might even do some good.
You can have "destructible walls" in Tremulous via AMP, but that's just an added layer if you are new to mapping.
You can create destructible walls without the AdvancedMap(ping?)Pack: If you use the "health" key on a door ("health: (default 0) if set to any non-zero value, the door must take damage (any amount) to activate.") and additionally set the "speed" key to something insanely high (I usually type a 1 with nine 0), you'd have a wall that vanishes when it gets hit.
There are downsides, though:
It only takes one hit to go away. But you could use several layers. I (temporarily) did that in a map, made a door that actually was 16 small cubic doors. That again can reduce FPS, but you should be safe if you put such a contraption into its own VIS area, away from the big arenas and bases etc. (which I couldn't do in my map, so I dropped the idea entirely).
You can set the "wait" key to something insanely high (1000000000) to simulate "forever", or you could have the shootable elements return after some time (which I did with my pixel doors, so if you weren't attacking them fast enough, the first removed elements would return, so all efforts would have been in vain).
You can select several brushes and make them into one door, works just like you would imagine, so a shootable rock formation can be done, too. Imagine shooting a rock wall down stone by stone.
Max amount of door-like entities (as opposed to speakers etc.) on a map is, afaik, 256. My pixel doors first had two layers, 64 doors each. That was a little much, but if you think of "arachnid2", which had no doors or lifts at all, you see how such a complex contraption wasting "all" doors can make sense.
Another tricky thing: Obviously, the shootable elements have to go somewhere. That's not really the hard part. Hard is when you realize that a door moving that fast can actually knock you through a 32gu floor. 64 should be safe, though.
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EDIT: "one hit to go away" - That's true. But I think that if you don't set the "health" key to 1 but to something higher, you could make sure that the element doesn't go away by a stray blaster shot but instead needs something bigger. Maybe you can kinda "filter" it for luci or grenade this way.
You could collect like three rock sound samples and give each shootable element a different sound. Example for one shootable element: "sound1to2" "sound/mineshaftofhell/rock1.wav", "sound2to1" "sound/null.wav", "soundPos2" "sound/null.wav", "soundPos1" "sound/null.wav". The null.wav is a default Tremulous sound, it's in the data-1.1.0.pk3