You can also surround the area with an AI Shape and set the anchor(in the properties panel) to combat territory or I think there's a protect this area anchor. The combat territory one will stop for the most part people straying out of the shape, and then you can set up a flowgraph that runs like this:
misc:start(or add a trigger of any other kind) --> then add an AI:Execute node and set the action to protect this point. Add another node called Entity:EntityID, connect that to the object ID port on the execute node. Then assign a grunt to the AI:Execute node and add a tagpoint(under ai>>tagpoint in the properties) put that at the center of the defence point, and asign that to the entity id node. I attached a pic to show, cuz this explanation was prob a little confusing.
Lemme know if you need more help with the flowgraph. This should work. Good luck!