|People are always asking for the combination to this puzzle. There is one, of course, but I don't know it, and it ruins the point of the puzzle if you do! Instead, let me explain what you're trying to do:|
The level is arranged in a giant 3x3 grid of rooms. The fuseboxes are just above the middle top room. You can see the divisions between rooms - each room has an outline of walls with lights in them. If the lights outlining a room are on, that room's power is on. If not, that room's power is off. Of course, the switches in the fusebox each apply to the room in the same position as the switch. Flip the switch one way, and that room's power is on, flip it the other, and the power is off.
There are two kinds of barriers to deal with: electric gates, which block you when the power to the room their in is on, and automatic doors, which block you when their power is off.
The puzzle solution is very simple - try to get where you want to go. When you reach a point where you can't get any further, see what's blocking you (and which room exactly it's in), and go back and flip the fuse for that room either on or off accordingly. Then keep going. At the most, it will take 4 trips back and forth to get anywhere you want to go. Notice that there are small passages that diagonally connect rooms as well.
It's impossible to get to the center room from anywhere except the bottom. But if you turn off the power to the bottom room (which you need to, to get into the center from it), you can only enter it from the left side. So the final path from the fuseboxes is down the left side of the level. You can also go down the right side to get a special weapon, but you'll then have to backtrack to continue on.