Always two there are... that summarizes this level nicely. Pretty linear, but don't forget to stop off in a side room near the beginning to get a Rocket Launcher so you can take care of the Zombies (plus it basically negates any threats you would have in the level due to plentiful ammo).
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Entering Invasion is a Single Player level in Q2. Turn left and collect the 2 Nails, then go...