Going up the spine

When you get to the pyramid structure up to the control room, you can bypass the path by instead walking up the central spine that starts way back out in the snow. It's easy to jump onto. You can take out enemies from distance with a pistol or sniper rifle or rocket launcher, or perhaps try some grenade work. All very nice fun, as long as you don't fall off. Eventually it's possible to get down off the spine near the top of the pyramid.

However, bear in mind that if you bypass the path completely, the usual group of Hunters and Grunts outside the door won't spawn, so the upper area will be empty and relatively boring. According to my tests, getting those covies to spawn requires you to enter the second tunnel and then the third (as you'd naturally do in normal play).

Note: At the place where the spine starts, there's a load of supplies in the crevice (a nice surprise when I eventually stumbled across them), and sometimes you might also find an Elite who got trapped there with his Ghost (not such a nice surprise). Sometimes even two Elites.