Oh Rats!

Try to stay alive as long as possible amidst a horde of ravenous rats.  These
rats will stop at nothing to take a bite out of you, even resorting to
cannibalism if the situation is right.  Your only hope is to stomp on them
before they get you.

Use the number pad with the num lock on to move around in the 4 cardinal
directions.  The rats have the added ability to move in 8 directions, but their
movements are very predictable.  Pressing 5 will descend down stairs to the
next level, where even more rats are waiting for your tasty flesh.  Every hit
from a rat will take away 1 HP, and the rat, now gnawing on a chuck of your
flesh, will no longer be a threat and will disappear from the view.  If you
move onto a rat, you will kill it, increase your score, and the rat will
disappear.