Defend Your Cabin

You have a nice cabin in the woods down by the river.  But now you must Defend Your Cabin because somehow the river got contaminated and now there are zombie bears running wild.  Don"t let the zombie bears get too close to your cabin.  Defend it at all costs.  To make it easier you have your guns, grenades and some mines to help you defend your land.  Kill enough of the zombie bears and you can upgrade your weapons and defenses between levels.