DO NOT READ IT UNLESS YOU REALLY HAVE TO!!!

1. In the second level, after destroying two automatic miniguns,
   you are in a room with many containers and headcrabs. Behind one
   of the containers there is a hole in the ground and a ladder (it
   is hardly visible, becouse it is dark)
2. In the third level (outdoors with many soldiers) there is a switch
   on the tower, which opens the hangar.
3. In the same level in the room inside the building (with stairs,
   a ladder to the roof and two soldiers) there is a hole in the
   ground and a ladder leading down.
4. In the same level after the elevator you have to push crates in
   order to reach the vent.
5. In the sixth level you have to find two keys. One is hidden in
   the attic of the railway station and the second is easy to find,
   so I will not tell you :). There are again two holes in the
   ground with ladders. One is hidden behind the building where you
   get the crossbow and another is in the restaurant.


PS> Sorry for my English, becouse it is a foreign language for me and
    I possibly made some/a lot of mistakes.