For context in the closed beta stone wall towers had a similar upgrade pattern to keeps and outposts with the tower starting with arrows and the option for mutually exclusive upgrades for springald and cannon.
You can still see this in the campaign for example in the 2nd to last Mongol mission where you play as the Chinese civ. The towers even have unique graphics to signify the upgrade.
However these upgrades were removed so now towers come pre-built with a springald and no option to upgrade to cannons in Age4. From what I can see this has led to a situation where stone wall towers are very strong early game.
I’m aware towers have been a topic of debate and while I am not a competitive player I think restoring the towers as they were would be beneficial as it would make them easier to balance. Right now the towers have no scaling so if they overperform in the early game but are weak later there are limited options.
With the upgrades restored the individual upgrades could be balanced seperately, the springald upgrade could be held off until Age3 or the Age4 cannon upgrade could be made cheaper or more expensive as examples.
I think removing the upgrades completely rather than rebalancing them was an extreme option and has left towers in a strange place where they cant scale up like outposts and keeps.
Unlike Outposts they started with an arrow attack but could get springald and bombard as upgrades. As for why it was removed I think CRothlisberger is correct.
Perhaps they thought since making them start with springalds was such a boon that removing the bombard upgrade was a good counter measure? But all it seems to have done is make towers far too strong in the early game and really weak in the late game.
The bombard upgrade was overpriced at something like 600 stone but why they decided to remove it from the game is a mystery to me.
It would be nice to have an answer from devs regarding this. I also played the beta and wondered why they changed towers. It was nice to build towers and chose in which one we want to invest in more resources to upgrade and on which ones not.