I am not taking arguments for costs and build time or some other form: so I am not rejecting your argument. Just think of my tweak in isolation.
Donjons: minimum range is 0 by default
The most noticeable difference between a Donjon and a Watch Tower is the size. The Donjon (albeit having higher HP is Feudal Age) is 2 times the size of a Watch Tower in surface area. In other words, it can be attacked by twice the number of units. It is also 4 times the size in area, so you will need much more walls/houses to wall off the base of the Donjon. This makes it much easier to take down in general.
Also the Donjon in Feudal Age can fire a maximum of 5 projectiles despite having 10 Garrison space, demarcating a clear distinction with the Teuton Watch Tower.