The water lock.
Losing Support is a written note found in Dishonored.
Transcript
Since we've lost the financial support of the Boyle family, things are dire and we've had to make major cuts. The patrol boats stationed in the Wrenhaven around the Tower have been redeployed to Northbank, to protect the few food shipments still trickling in from the farmlands outside Dunwall.
Sir, I'm warning you now, this means that the approach to the Tower from through Water Lock is now exposed. It's unlikely that an attack would come from the river, but I know how you worry.
Ironically, there's one thing working on our favor: Since the Water Lock malfunctioned last month, that idiot who maintains the moat flow controls hasn't been able to repair it, and with Sokolov missing, no one else is sure how to get the blasted thing working again. So anyone conducting an assault from the river would have to climb up the pipes in the Water Lock, which is hard to imagine.
Location
It can be found in the war room of Burrows' safehouse during the mission, Return to the Tower.