[@gcold], A question if I may.  This Redoubt, as far as I can tell, does not exist in canon on any map I have been able to find.  So, what do we know about it?  Has it been scouted before?  Is there a watercourse outside the walls?  Are there caves nearby that might allow entry from below?

So far it simply seems that we know that it has a garrison and walls.  That isn't much to go on.  Anyone patrolling the road would have seen more than that, just in passing.