First question is to issue it normal move commands on a region waypointed, if you want it to slay your unit if it gets too close, have the editor run proximity checks around the region of the unit (or the hero), then auto-kill the hero or order the unit to kill the hero then resume its course.
You can set the tower to have an artillery attack and constantly shell an area, or, you can make it allied to a player until they enter a pre-determined region and then have it go hostile and start firing.
I believe patrol is one of the standard commands you can give a unit, along with move and attack move.