Fortress or War Wagon.

[Dagravian]

Basically a reinforced wooden wagon with someone throwing something from it... Could be rotten tomatoes, rocks, Arrows or any gunpower based weapon... Unlike the current wagon, this wagon will not carry any aditional unit, but it will have its own atk power and range.

I'm not sure about ideal stats, but we can start with the current wagon with 4 range and upgraded pierce armor. It should have bonus vs heavy calvary, light infantry and ranged infantry.

The basic version can start with crossbows, spears or javelin throwers. (Or Any arrow/spike based weapon...)
The second upgrade it will have a couple of arquebusiers or musketers. (Or Any early handgun alike weapon...)
The third upgrade will carry one or two cannons.

Or if we go for the 2-step upgrade for this, we should stay with gunpower based ones...