This game is divided into nine sections. If necessary, you can type+ to warp to an individual section within the game.
As you begin the game you'll be thrown immediately into the action. Backup to collect the ammo directly behind you (if desired), then clear out any four legged walkers that are close by. Grab the health power-up if necessary, then walk near the large door. It will rise up to reveal the first corridor.
Enter the corridor and grab the second gun. Remember to conserve ammo this weapon will fire quickly. Continue forward and up the stairs until you reach the next door.
After the corridor, you should enter a room with ammunition along the side as well as a new weapon. This weapon is the first explosive weapon, and is located on the far right corner. Jump over the railing to collect the weapon and ammo, and exit on the far door to proceed.
You will encounter a large spider-like enemy in the next room. Shoot it twice with the launcher to kill it. Once you are about half-way across the room, another spider enemy and various small walkers will appear.
Collect the next weapon. It is a single-shot rifle similar to your starting weapon, but inflicts twice as much damage and uses a separate ammo pool.
Just before the cooridor turns upward and to the left, a giant robot will begin to advance. Shoot it with your launcher or energy rifle; unlike other enemies, it will take a significant number of hits to destroy.
After taking it out, proceed up the stairs and collect the ammo laying about.
The second floor starts with a long corridor which turns to the right. You will encounter a large number of walkers, a new enemy with two large horns, and a giant robot walker. After the walker is another set of small monsters similar to what you saw.
Later on the second floor, you will encounter three spiders, followed by a large robot. You can also collect health if you took some damage in the previous area. While the corridor is tight, it is possible to run past the giant robot.
In the next room, you will see a straight upward staircase, as well as the second launcher and its ammo. When you collect it, you can proceed upward to encounter another large robot as well as various smaller enemies.
You'll finally enter the room with the pillar that was shown in the introduction video. Head up one of the ramps and hop onto the center platform to finish the game after the platform descends to the floor.
Alternatively, you can walk directly into the pillar, since the pillar itself doesn't obstruct movement.