Mission 1: Operation 627About a third of the way into the mission you'll enter the inner workings of the Russian Gulag. While inside a maintenance tunnel you'll have to open a grate that leads into the panopticon (cell block). Charlie squad will kill the lights and you'll have to rappel all the way to the bottom floor. Ensure you kill all the guards (carrying torches) as you slowly pass each of the windows.
Guards:2x Staircase
3x First Window
3x Second Window
4x Third Window
5x Fourth Window
5x Bottom Floor
Check out my video guide for more detail: