Run straight ahead and on to the elevated plateau like geographical landmark in front of you. Take out the guard patrolling the plateau. You may try to sneak past him by hiding behind the trees, but that will make you a target of the snipers positioned at the tower that commands a good view of the plateau. By taking the guard's clothes, you can disguise yourself and avoid becoming a sniper target.
The mission requires that you do not be questioned by any guard, since being questioned and asked for an ID will most certainly blow 47's cover.
Keep going on and make our way to the right front side of the mansion. There is a guard patrolling the fenced power unit that needs to be switched off to make sure the tripwires are off, if 47 is to make his way inside the mansion. Although the it may appear that the guard needs to be killed, and if it is done silently, the rating is not affected, there is a way in which killing him can be avoided. Sneak to the front part of the fence. As long as 47 is behind the fence, the guard cannot see him. As he turns around, follow him closely and turn left, open the gate and switch off the power quickly by running, and run back out. The fence masks your movement. Then slowly make your way back to the back wall of the mansion. Avoid being seen by the guard patrolling there.
Continue to the other side (right side) of the mansion and time yourself to enter the mansion from that side without being seen by the guard there. Head to the right immediately and through the door there (disguised as part of the wooden wall) and up the stairs. Sneak through the door right in front of you, and turn right. Just up ahead is the other power unit, and it can easily be switched off.
Hide yourself so that the guard that comes out of the door doesn't see you. Then go up the ramps located to the left side of the door you just came out. Go through the door, and make your way to the end of that particular corridor. You will find the last power unit that needs to be switched off.
Go out the door closest to the unit, and avoid the guard outside turning left and left again up the ramp and to the right side where the entrance to the mansion is. Up the stairs, and this brings you to the entrance of the main mansion.
Silent Assassin Method
Getting a silent assassin rating for this level is tedious but not too hard, it will be a breeze in comparison to the hidden valley. There is however only one proper way to do it.
-Go to the ridge on your right and equip your aneasthetic, sneak up to the lone guard using the trees as cover and sedate him. You'll need his uniform to get past the sniper fire up ahead. Not sure if it is possible to evade this without, but it would be both tedious and useless to do it.
-Run past the guards to the west of the castle, Diana will brief you on the generators. There will be one guard patrolling, walk up on him with aneasthetic and sedate him while he's checking your ID. Then deactivate the generator. The beam grid at the front door will now be unlocked.
-Go to the front gate and walk to the right door. If you run, the guard above you will look your way and make it impossible to sneak by. Enter the door you see after ascending the stairs and you'll see the second generator.
-Alternatively, you can go around the castle exterior to the North-west gate. Don't go through the north-east gate, it's a death trap for the sake of a death trap. Just an entire battallion of snipers that will see through your disguise. West however, there will be only one sniper who doesn't care what you do. If you brought the crossbow and don't care about silent assassin, you can even shoot all three guards at the front gate without him caring. (And this is possible without an alert; go to the gap in the wall, shoot the patrolling guard when he sticks his head out of the west door, and then headshot both guards. Their pal will only start to spin nervously. You can then descent through the gap.) In order to get past the two guards at the windows however you'd need to get them to face away by running at the gate, rendering the process useless.
-Go up the stairs behind you and through the corridor. Don't worry about the ninja's in the support beams, they won't care even if you run. Deactivate the last generator.
-Sneak past the guard at the courtyard and head to the exit. You can double-back and take the route through the North-west gate to a now deactivated laser grid if you want to avoid him more easily, but this route is again rather rudimentary considering it shouldn't be too hard to get past this last guard.