Having only one completed lookout tower early on, I disabled both slots until I can have the income to support them. The tower UI shows 0 Au monthly cost once you do this, but the resource bar at the top continues to list soldier training (-5) as a monthly expense even a year later. The People manager UI shows 0 soldiers and 0 guards.
This is not a bug, towers have an upkeep cost regardless of employment.
You can toggle production at the tower to disable it and remove this cost.
Then the “monthly cost: 0” listed in the tower UI is a bug.
That I cannot reproduce, how did you get it to display 0 without being disabled?
This toggles production.
I stand corrected. It was another building I couldn’t disable, but yeah, I already disabled this and the cost is still listed in the top bar.
I disabled both employee slots first, then disabled the building, if that matters.
If you disable production, the upkeep cost on the top bar takes a month to update as that lists the previous month’s costs.
If production was not paused however and the tower UI still listed a cost of 5, that is an issue I would need a repro for.
Over a year since I disabled it, and even just now I watched as the time lapsed into yet another month.
And this is your only tower?
At the moment, yes. The rest are still being built. This is just to capitalize on the tower exploit until I need to use them, then moving will be quick.
If I ask to see your save and find a lone tower somewhere, I might have to be annoyed. 
Let’s see the save please.
Even zipped, it’s over the 4 MB (4,194,304 byte) limit. Spanning across two files is possible, but the file extensions would need to be changed to allow uploading to this forum. I just started the game today on the updated version.
Simplest solution is to upload it to a file sharing service, such as Google Drive. Thanks.
Another reason to add a feature that lets us cycle between all of a specific building type, jumping the camera as well as showing the UI update. Will make it easier to determine if there is such a “lone tower” out there. In this save though, the only completed tower is just up the hill a bit from the town center.
Also the .map file please.
Oh, it finally removed the soldier training. I don’t know what took so long.
Things do seem fine when loading your save and toggling that tower.
If you find a way to recreate what you saw, please let us know.
Maybe I was mistaken about how long I had played after stopping production on it. I just watched a video recapture after toggling production and it seemed to take about a month and a half to update the income and expenses window, which is tolerable I guess.