It was really fun playing this game. I also played the Age of War games quite a bit back in the day :)
Here's a few notes or suggestions :
- The units' lifebars are not always visible when there are a lot of units packed in an area. I'd suggest making them render always in front of the character units (I don't anything about Godot, but in Unity you would use the Sorting Layers for that). And maybe above the units instead of below them ? But that's more of a personnal preference really^^
- A nice addition would be some tooltips or other way to display some extra informations about the units and the upgrades so the player knows exactly what he's choosing. For example, I had no idea what the Rogue did until I read your devlog. And I'm still not sure what the upgrade with a lighting symbol does. Increase movement speed or attack speed ? Or something else ?
- Taking things one step further, a whole stat sheet system would be great. You could see the stats (life, damage ...) of each units and their increase when you purchase an upgrade.
- For the buttons spawning the units, an option (like a sub button or something) to spawn the maximum number of units you can all at once would be great. A strategy I would often use would be to wait as long as possible and then spawn all the units at once so they'd stay grouped up together.
A solid entry overall, good work !