The power can be activating by following some tunnels to a subterranean room. After knocking off the guards, activate the lava flow to cause power to flow through the lines. Head back up to the surface, and open the now-active door, then kill the guards in the room behind it, watching out for rear attacks.
After this room, you'll find yourself outside again, with three routes to follow. The rightmost leads to a filled lava area, the center to a outdoors area, and the left to corridors overlooking a lava flow, as well as a save terminal (which you may want to use before continuing, but watch for the guards there).
Head down the central corridor, dispatching the drones and Troopers that lie in wait, and destroy the switch that overlooks the lava-filled room. The lava will drop, allowing you to use the rightmost tunnel mentioned above. Head back to this passage, and follow it down and into the once-filled room, then to the large lava flow with narrow walkways down the middle. Follow this to the end, and collect the stash of goodies there, then turn around, and look up and to your right - you'll see a switch that you can hit to open a door near the save term.
Return back to the save term area, and, after saving, proceed to dispose of more troopers and guards. You'll approach a hallway, and the left branch leads to a large open chamber with a narrow stairwell and a Jugger. Take out the Jugger with your fusion pistol, then carefully navigate the stairs, watching out for the troopers both on the stairs and in alcoves above them, and Enforcers on another alcove. Behind the Enforcers is an "L" switch, which, when hit, opens the door at the bottom of the stairs.
Go through this door, and you'll come to a narrow corridor with small openings into the room below. At this point, several Hunters and Enforcers will engage you - hang back in the corridor and you can take them out one at a time. A switch at the end of the corridor will open two doors that contain more baddies as well as switches that you need to hit. After clearing the area, hit the two switches to open the gates to the next part of the level.
Following the hallways, you'll come to another save term and comm term, next to the entrances to a major sub-chamber of the volcano. The ledges of the volcano are spaced that you need not take lava baths maneuvering them, but the Jugger and several Hunters and Troopers might not you in. Using a 'fire around a wall' approach to clear the way, make for the large 'column' structure in front of the lavafall, and hit the switch in here to activate an elevator down below.
You can get down below either by making a leap of faith over the falls, or using the elevator that is located to the right of the falls. Once down there, the elevator that you just activated will take you back up into the heart of the complex.
There will be two Enforcers guarding this way. Get rid of them, then, after refueling and saving, proceed to where an airlock is located in order to enter the main stronghold.
You'll notice that there has been no mention of the 4 rooms that have 4 switches each. Hitting a switch deactivates them all, and in some cases, will activate something *bad* for you. These 4 rooms, together, are a main secret on this level. You need to hit the right switch in each of the rooms, and, in order, this are switches located in the south, east, west, and north positions for each successive room. Once done, you'll open access to a VERY large ammo stash and an interesting side story to the marathon plot.
The "circuit light" trick is rather old. Just the use of the "phase" variable in the Light box to offset the lights appropriately.
The "lavafall" that is started near the beginning of the level is a very fast platform that lowers itself, revealing a static poly face with the vertical slide of lava.