The quest that is generated also does not seem to include any kind of minimap marker or waypoint pip. These attack events generate a quest in your tracker, and to my knowledge, the sole condition for success of the mission is the complete elimination of all enemy forces spawned by the event. I haven't seen any attackers change zones or run off yet from one of the attack events, though it may be interesting to do some testing with the Yellow Belly Syringes.
From what I have seen from my tower, the cellar entrance is somewhat near one of the approach vectors that Raiders and Gunners use during an attack, but you should be able to block potential access to it by using some kind of outward-facing fencing or walls.
