I was having issues with this for a day, after searching high and low. I spent enough time, and I've an idea for you to tinker with. I haven't tested anything, but assuming you know what you're doing, take a look at NPC -> ArlingtonLibraryRaiderGunGuard1 /2/3. I haven't checked, but this appears to have a package that says "Stay at Position"... I've successfully figured out how to make npc's stay IN a cell during combat, but if the stay at position does what it says, you could set a guard package with like 0 radius (assuming the npc won't move, but can search for enemies down below?) and the stay at position package would keep the npc inside the guard package? Anyways, if your npc is inside a water tower and it's a cell, like a house.. Then create a package, and set the package type to "guard". From there it's self explanitory but I just simply set mine to a cell, so it would stay inside. On the flags tab of the package, check the "continue during combat".