In lot of other succesful AoE/AoM franchise games, the techs to upgrade walls and towers are not behind university but rather done from the building itself. So my suggestion is following:
Fortified Wall can now be researched from Stone Walls (Price increased by 50F/50W for balance reasons)
Guard Tower and Keep upgrades can now researched from Towers
Arrowslits, Murder Holes and Heated shot still in University
This opens more space for adding couple new techs to university while removing some building requirements for using towers and walls effectively.
A new building that would proceed the Stone Wall can be introduced and it can be available in the Feudal Age while the Stone Wall is moved to the Castle Age and the Fortified Wall moved to the Imperial Age.
No thanks with the Imperial wall because that isn’t needed. Just need Light Wall, Stone Wall, Fortified Wall for Feudal, Castle, and Imperial Ages respectively. That’s all. Rather than reducing the HP of the Stone and Fortified Walls, they can either have their HP increased or become more resistant to damage that those walls take.