I love what you have going on in TDD Duel! At first read it makes me think of it as a twist of Kingdom Rush, Clash Royale and Bloons TD 6 Vs. where the twist is your custom pathing to the enemy base.
Your GDD is extensive but good at hitting most of the design features of your game. It's specific in its description of user interaction and menu navigation which leaves little to the imagination there; it's a good thing it does. The one thing I had trouble piecing together when I read it was exactly how you create your custom paths to the map border edge.
Do you just build it up to the edge and units will march off/attack from there? Do some units attack earlier along the path or later rushing down the enemy base? Are there units that might ignore the pathing once spawned and require a special kind of Defender, like a Flying unit that just goes straight for the base that's weak, but needs an Anti-Air style defender to gun them down? Can players in a game of TDD disrupt or interrupt each others path? Can they use sabotage powers or abilities to make a path through an opponents path? What kinds of map design features will prevent a player from just building "the shortest path" to the base each match to make sure their troops consistently get there first?
Bombardment of wondering aside, I'd add this game to my library for the challenge and tower building aspects. I like fantasy, sci-fi and tower defense games so this is a great all-in-one package for someone like me!