There was an explosion at the base and all experimental creatures broke free, and some of the personnel were infected. You have barricaded yourself in the Tactical Retreat and must not let the sick and mutants through the fence. Destroy overly large creatures, like spiders the size of a cow, as well as people in red overalls. Let those in yellow pass, so you save the surviving scientists and they can stop this nightmare. Be smart and attentive. Change weapons to more powerful ones whenever possible, because the number of those who need to be killed grows in Tactical Retreat.