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 you're 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, duck (to go slowly) and walk to the edge. Once you start falling off the edge, to the ground, 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.

Alternate second glitch: Drive the Buggy Vehicle near the right side of the right tower, if you look towards the dam from middle of the battle field. Park it right next to the wall, then get out while pushing left. You should then be inside the tower. If you lean out a bit, you'll be able to shoot.