Yeah I suppose you could use a Stone Tower with high quality metal/steel lock outside of a palisade provided one can afford the material cost but buildings all have to build up their soak values making it easier to destroy for awhile. However in world 14 stone tower materials were changed to include glass panes in the recipe and any metal or hard metal, forgot which it used to be, changed to steel bars. I've not made any glass panes this world and I think the stone tower still requires those so I can't say for certain if steel is also still necessary. Plus just having high soak doesn't guarantee that they won't be broken by someone at some point if they are outside of walls, unlike a palisade which you can't hand bash regardless of strength when it has full soak.
Apparently the water mills have 12 soak, but that's according to its wiki page as well. Better than a cabin but not much better than a stonestead, and the same as a mansion.
I'm unable to find any patch notes referencing the change to material costs for stone tower and some of the other buildings, let alone anything that might reference if it was reverted. It doesn't seem like they mentioned that in any update thread but I did find this unofficial list of changes which mentions stone tower (thanks Vesena):
https://www.havenandhearth.com/forum/viewtopic.php?f=42&t=72867