If you want to play the game on your own without any help from
the walkthrough, here are some tips that you should know :
Always remember where you saw locked door and you have no
keycard. You will come there again if you have the keycard.
Always check boxes that forms some kind of stairs. You might
be able to climb it.
Always search one area thoroughly, then move to the next area
after you sure that you do not left anything there. This way you
can save time, and you won't get back there to look for things
that you haven't found.
You won't hold 2 keycards at once. Once you find a keycard,
you won't find any keycard until you use the one you found on
a door. With this knowledge, you should know that if you cannot
find the keycard to the door, concentrate the search the areas
from the door you just opened to the door you can't opened.
If you have a mission of collecting a keycard, kill all terrorists
and scientists, then search their bodies to find them. You won't
find a keycard in a box.
Improvise - that's the keyword. The stage design is very good
in this game. So try everything that crosses into your mind, even
when you thought it's not possible in a game.
Check your radar. Sometimes it shows the item needed to finish
the game, like the viral bomb or etc.
If you can't accomplish a mission, try completing the other
mission first. Sometimes it is needed to accomplish the mission
you're stuck. (Hey, you cannot accomplish a mission at a particular
place if you don't have a keycard to open the door leading to
it ^_^)
Just remember that the stage designer of this game seems to
give you hint about where to go. Such as : if you don't know where
to go, sometimes an enemy will appear from a ledge. Maybe you
should climb the ledge where the enemy was.
Use flashlight to find hidden boxes. Sometimes you feel that
the place is bright enough, but use your flashlight to be sure.
Use the aim button (L1) to look around the area. Look up or
down to find ledges, iron bars, or something. Use the viral scanner
and flashlight to help you.
Before going down from a bridge or ledge, check with the aim
button (L1) to see whether the ledge is too far. Climb down if
it's too far, because if you jump, you'll die.
Use roll if you see a hole on the wall. 13. If anything fails,
kill all enemies and search the place thoroughly with flashlight.
If Tips #13 doesn't work, walk to every place and press Triangle
wherever you walk.
If all the tips fails, hey.... you can read the walkthrough
^_^