It'd be cool to see more of this! There's a lot of really good stuff going on. I like that you tied the ability progression to the distance traveled, as opposed to time or anything, which works really well with the upgrade system. It makes upgrading a basic troop useful for simply traveling more distance, even if you knew they wouldn't be able to actually make it across. On top of that with your own monsters dropping coins, it really well designed.
Using multiple spawn points to adjust for the cool-down so you can double up the same type of troop is a design thing you had in there too that I really liked.