You need to go through tunnels and secret chambers, cross the yard and then climb over the wall. Stealthily take down the guards from behind, but be careful that they don't hear your running footsteps. Make sure to avoid the searchlights to hide your shadow. Control keys: Walk: ← →; Run: shift + ← →; Jump up: ↑; Crouch or crawl: ↓; Leap forward: space (spacebar).