If you are playing as the Terrorists, speed is the key for the first part of this mission. Run as fast as you can down to the Security section and destroy it as soon as possible.
The balconies are great for covering the ground floor, for either side, but beware enemies sneaking onto your balcony and attacking you from beind.
The spawn point for the Counter-Terrorists in this level is particularly vunerable. A well timed LAW can destroy an entire team before they even have a chance to get their bearings. Timed and Remote C4 can also be used effectively toward the same end.
If you are playing as the Terrorists, and you get the Cross to the extraction point, drop it on the ground. Your opponents cannot pick it up, so leaving it there isn't a risk. It also allows the person who was carrying the cross to get their weapon back out and help defend the area while the helicopter approaches. Once the helicopter arrives, all you have to do is run out and pick up the Cross for an instant victory.
If you are playing as the Counter-Terrorists, it is extremely effective to hold the long staircase and hall leading up from the Terrorists second spawn point. There are many positions your team can hold to fire on the enemy from relative safety. Directly behind the staircase, where you can fire down at the backs of your enemies, is particularly effective.