A thing from Dead Simple - kill all enemies

How do I do the thing from Dead Simple:
- you kill all Mancubi, wall opens.
- you kill all Arachnotrons, you can trip a switch.

Can you help me?

Use ACS scripting. (harhar)
For the vanilla method, tag the walls to be lowered with some special number, 666? For the stairs to rise, also give them a special number. Look those numbers up by opening doom2's map07.

Or use the MAPINFO lump to specify some kind of special, can't remember right now.
Also, the 667 tagged sector rises to the height of it's below texture.

The special linedef tags that open/close things in Doom2 only work on certain levels, and as far as I know only on map 7. On map 7 using sector tag 666 will lower a sector when all mancubeses are killed, and sector tag 667 will raise a sector when all arachnotrons are killed. There is a sector tag you can use (can't remember the exact number, maybe it's 666 again) that you can use on any level that opens a door when all commander keens are killed.

The only way you can change these properties with the original .exe is by doing some dehacked. But with advanced ports, i.e. zdoom, you can script whatever events you want (e.g. opening a door) when certain monsters are killed. But I suck at ACS so can't tell you exactly how it happens...