From The TimeSplitters Wiki

Siberia Tower Glitches
Requirements	TimeSplitters: Future Perfect
Finder	- Abyssion180 - BboyNaishun

Glitch: In siberia, if you go to the tower (the one with the bridge leading to it, but no ramp) and, right before you enter the place, (in the doorway, if there was a door) you walk into the wall where the "door frame" is, you'll go right through it, and can travel inside of the wall! (no one can shoot you if your all the way in: perfect sniping range for killing the blue team)

Second glitch: If you've done all the things listed in the first part, and your inside the wall, if you duck (to go slowly) and slowly walk off to the edge, and, once you start falling off the edge, to the ground, then try to move in the direction of the building while falling. You should be in another safe spot (although you can't shoot outside until you go there) inside the tower itself, pretty cool!

Alternate second glitch: Drive the zeep near the east side of the east tower, and park it RIGHT along the wall, and get out, while holding left. You should then be inside the tower. Lean out a bit, and you'll be able to shoot.