I saw Marrowkai Minion's post with an awesome looking tower art. However the described enchantment didn't really place any tower (or maybe it did but the main thing was empowering castle archers rather than building a tower)
So here's an idea about building watchtowers:
Watchtower is a unit but to place it you need to have a giant builder. Enslaved giants get an ability to become a giant builder. When the ability is used, enslaved giant will go to garrison and become a giant builder forever losing it's ability to do damage. You can cancel this process by giving individual commands to an enslaved giant on it's way.
When you have a giant builder, you can place towers wherever you want (but not on top of your statue or ores).
Statue consists of floors which I tried to draw. As you can see those floors have stairs for elevating and holes for shooting (The door is in the back I guess). One floor costs 500 gold. You can place those floors on top of each other to create some real watchtowers or just build little houses next to each other. When you decide to place a floor, giant builder will slowly go to selected location and place it (it gets those floors from somewhere in garrison).
One floor can contain 4 units. To move your forces into the tower you need to use hold command on units who are coming near the tower. They will automatically fill the highest floors. Units who doesn't fit into the tower because of their size, flight or if there's just no space will just stand near it. From now on you can click on individual floors and command units inside to go upstairs or downstairs (which on the lowest floor means leave the tower). Units inside the tower will ignore global commands.
Archers and archer generals will shoot from the tower. Each level of elevation grants shooters 25% more range as well as visibility in those locations. However it creates some blind zone near the tower where they can't attack. Melee units do nothing in their rooms.