The concept is simple: get to the end of the level, descend to the next floor, rinse and repeat. But simple does not mean easy. Each level is swarming with enemies, and your ammunition is limited. So limited, in fact, that you are often forced to retrieve every bullet that your fire. But itís okay; bullets arenít destroyed when you fire them, and theyíre easy to spot. But there is a good deal of strategy involved.
Picture this: an enemy lies in wait around the corner, in a field loaded with turrets. You have one bullet. If you take a potshot at the gremlin in the field, you will be defenseless against the turrets. But if you get its attention and wait until itís near before blowing it away, you can just retrieve your bullet with no pressure.