If you don't like to know how to pass levels, don't read the following script.

Lara starts in a motorboat, she goes to the shore, she pulls the lever in the left wall and enters the house, she climbs the wall and goes to the 2nd floor, there she finds a key.
She gets out, and goes to the far end of the path, there is a keyhole there, she puts the key on, then she climbs the wall, and when she reaches the end she unclimbs, and then gets hold again, and instantly press action to climb, then crawl till the end of the ventilation tube, unclimb, and pull the book/switch, open the door, (these doors have got knobs, unlike the grey metal ones) and go to your left, in the room that follows, there is a pushable block, pull it, till you can get inside and get the key, then go to the previous hall and go to the right, you should see a keyhole here, out the key and go to the newly opened door.

Now, there is a hall in here, at the first intersection, go to your left, and then open the door, you are in the bathroom now, then open the leftmost door, go to the bottom of the cubicle, and then look to your left, There is another vent here 
(The office where you go to must have a terrible smell) climb, and go through it, then at the end, unclimb, pick the cookie,(That’s what you are looking for, and sorry for the invisible wall, but that’s because of the door=not my fault)and pull the lever behind the plant, now, get out, and go to the spiral staircase(at certain point there is a weird grey wall texture, that is because of an unfortunate intersection with the vent, sorry) and the door to the terrace should be open, go to the opposite side of the terrace, and to your right, you should find a space to crawl, go through it, and then complete the jumping course, when you are at the top, pick the key, now go back to the terrace, and go to your right, there should be a keyhole in here, out the key on it, and go to the newly opened trapdoor, go through the tunnel and at the end, climb, you should find the underwater course here, get in the boat and go through it, don’t stop when you reach the other dock, continue till the end, get down, and climb to the space where you see a bar in the ground, pick it, then get inside the boat again and then go to the dock, get out of the dock and use the crowbar on the door, now, climb the wall,Use the same method that in the first wall, now go through the tunnel and descend, so you are in a kind of temple, now to the right, you'll find some doors, go to the first to the left, and press the switch, complete the time course and now climb the wall, go to the balcony and jump to the top of the building, then climb the wall and jump backwards at the top , pick the key and go down, now go to the left wall, and then at the border of the maze, jump and get hold of the ceiling, Go forward untill you find a space, unclimb, and then pick the key, go back, and put both keys on the keyholes on both sides of the building, now enter, and pick the crowbar, get out and use the crowbar on the door on the back of the building, now enter, see for yourself the surprise and finish the level!