Nice little game!
I had briefly thought about the idea of a tower defense game but reversed, but it didn't seem like it would had much depth. I could only think about the previous GMTK game where you had to avoid your own defense shots. But, you made me realize that there could be a lot more to it!
Placing roads was an idea I just hadn't thought of, but it makes a lot of sense as the main framework of how the game works.
I think there is a lot of room for more mechanics and ideas here as well, I kept thinking about how in BTD battles, you can purchase the waves sent to the other player. Maybe it would be beneficial exploring into the ideas of having to buy your own waves, giving the player control of what they send out.
There was also a bit more depth to the road placement, but I think there could be a lot more added. Most of what this was is : Place roads away from turrets, create faster roads where they are many turrets. (If we ignore the exploit of placing unconnected roads to spawn towers away from the real road...) I wanted to create branching paths, but found it less useful since the tanks go in seemingly random directions. Maybe you could add a feature that allows for you to add logic to your path, (maybe costing a price) that allows for the player to add controls like: Every other tank goes, only X color tank goes, Send for half wave, etc. I think this could open up a lot more thought when having to design a road structure. It may also make the player need to deal with throughput?
Having more tower types could also lead to more dynamic paths and creations. Maybe ones that have piercing shots, but can only shoot in 1 lane, which would prevent long stretches of roads. Currently, there is also no real reason to have a slower path other than to save money. Maybe there could be some tower types where it would benefit to move slower.
Not having time to think about my road placement was a bit stressful, since I wanted to create something that was logical, but was only given 10~ seconds to do so. I would dabble with the idea of a keeping a forced wave timer or not. Since it makes sense to have the waves out of your control, since it is the opposite of the Tower Defense game. But, it may be more beneficial and fun to make it so the player can take their time and send waves when they want.
Some other QOL features would be nice, like being able to place better roads on top of old ones without needing to dig them up first.
Nice job!