I suggest instead of Harvesting after shields Harvesting should be after construction (farming line) or something and another tech would be after shields. This tech would be "Engineering." No, I don't suggest bringing back towers. A new tile improvement would be unlocked, a Fort, and can only be built once in a city. When a unit occupies a fort, the unit's defense and attack increase. When an enemy melee unit takes out the occupying unit, the enemy unit moves into the fort and takes it. When a catapult takes out a unit occupying a fort, the fort is destroyed (boulders take out walls, therefore no more fort).
Another tile improvement could be unlocked (unlocked by farming), the Orchard, and could regrow fruits for cities, but only one can be built for each city. Every *insert turns here* it would grow more fruit
Now is more terrain. I think rivers would be a good thing for the game. Rivers are on the border of tiles (not every tile). Rivers impact movement and defense and attack. When an attacking melee unit (A) attacks a defending unit (D) over a river, A's attack is lowered and D's defense is upped. When being invaded, players could use rivers to their advantage to stop enemy units and create a defensive line.
New units:
Spearman - super effective against knights (anti-cavalry)
Crossbowman - upgraded archer
Ballista - upgraded catapult but still have to set up after moving
Thats about it.