it prevents enemy mg from sneaking in and gives los
That's what happened to me heh, since it was a church and churches have the best protection (+10 to holiness)
Some other things I thought of that I've learned since playing.
After a loss, you still gain 1 CP, 1 resource bonus, and 3 xp to all units.
Sadly I don't know what happens with a win just yet =(
Also what does the Officer do? And I'm guessing you can only buy one?