Towers and fortresses are used in 1v1s and TGs on mainland.
Walls are not but that's because Mainland is as open as it gets, on top of players sometimes not realizing they are in "must defend" situation. Also, that champions are weak gives less incentive to stall out.
Mainland is not the only map. There are maps that are less open but not as closed as the ones you created, and they give some easy to defend positions, one example are water maps (eg Rivers, Corinthian Isthmus). I know by experience they play like this
At the end of the day, players mostly play TGs on Mainland. It may be due to lack of taste, but the fact that many options are hardly viable for civs doesn't help.
I kind of agree. In AoE2 all civs have trebuchets. In Delenda Est I think infantry can build catapults on the field which is an interesting idea.