Upon starting the mission, hop the fence in front and use the stalking zone and the sugar cane field to move up towards the buildings ahead. When you reach the end of the first sugarcane field, when it is safe to do so, cross the street and enter the next field. Make your way over to the right hand side of the building ahead. From here, use the stalking zones along the wall to move up and out of line of sight.
Work your way along the wall behind the building to the south and enter the stalking zone bush here. Peek around the corner towards some stairs. At this point, activate eagle vision to locate the guard with the key. The good news is that he will patrol close to your location, so as he comes close whistle to him to attract him and assassinate him from cover. Loot the key form his corpse.
Cross the street into the sugarcane crop on the other side and make your way up to the wall to the left of the stairs. Climb to the top of this and hang from the ledge. Wait for the guard above to stop above you and use a ledge assassination to put him out of business. Climb over the wall and enter the stalking zone ahead.
Now use the bushes along the wall to the left to work your way towards the objective marker. Make sure to kill any enemies on the edge of the stalking zone with an assassination for the optional task (we need to kill three baddies form stalking zones) if they come too close. When you can go no further along the wall, climb the overturned cart here to access the wall above. Jump over the ledge and immediately enter the bushes in front to avoid being spotted.
There are three guards below the tree in front of you and to the right. There is also an open structure to the left with some crates alongside it on the side closest to you. When all three guards are not looking (or you have whistled them over and dealt with them one at a time), run over and simply use the crates to mask your movement. Keep sprinting to the objective marker because there are no guards past the wall. Interact with the door here for a scene.