Gate Keepers is the 6th mission of Crysis 2. For this walkthrough, players will start out by navigating New York City rooftops, and then fight (or stealth) through heavily fortified CELL positions, in order to gain access to the Church tunnels.

1. Head towards the objective via the rooftops. There won't be any enemies at the beginning, giving you time to freely sprint and observe what is happening in the air.
2. Note: make sure to jump over the spaces between the roofs or you will die!
3. When you see the fenced- off rooftop, near the two water towers, start making your way down the building via the balconies below. Make sure to drop down steadily, or you will take too much fall damage and die. 
4. Note: When you don't see any more landings below you, take the metal "elevator" anchored to the building.
5. Start heading towards the objective. Jump through the broken window into the shop on your right to avoid the soldiers, and resupply on the ammo inside before moving on.
6. Stealthing along the edge of the road, and then through the second shop on your right to avoid combat completely is a wise decision here.
7. Continue heading north, around the soldiers, and stealth around the armored car in front of the Command Center gates.
8. Go into the building behind the armored car, and wait for a mission update.
9. Kill any soldiers that discover your location, and head out of the building towards the new objective. Once again,being stealthy here is a good choice as the area is crawling with soldiers.
10. Go into the building just around the corner, and move up the stairs towards the objective.
11. Jump into the large open area with the scyscrapers and continue on.
12. Go up the stairs around the fence.
13. Stealth kill the soldier on the roof.
14. The area below is swarming with soldiers. Stealth down towards the nearest objective.
15. Activate the fuse by pressing F and head out towards the next diversion objective.
16. Light the second fuse, and head out back the way you came.
17. 2 soldiers will be coming out of the Church. Kill them and head into the Church.
18. Go across the Church pathway towards the objective.
19. Head into the CELL trailer and activate the objective. Take any weapons/ammo you like.
20. Head back towards the Church.
21. Drop down to the entrance into the Church, near the objective marker.
22. Drop down again until you see the fenced off room. Shoot open the fence and press F on the objective.
23. Head into the tunnel.
24. Don't worry about the alien. Just run after it into the next tunnel to finish the mission.