Me likey
Surprising depth in tower placement: the best placed tower is not necessarily the one with the most road in its range (as in top-left, for arrow towers.) Even though arrow towers have most road-accessibility in middle and top-left, they are most effective in top-middle!
Also, having the interest compound after each wave creates depth. So far, I think I have a sequence for the first eight levels to maximize interest (these assume optimal placement):
1. 2 arrow towers (at)
2. 6 at
3. 10 at
4. 11 at
5. 15 at
6. 18 at
7. 20 at
8. 20 at
Upgrades must be avoided for arrow and fire towers. Earth, cannon, and air towers must be avoided altogether.
Question for the creator: what is the algorithm for the final score?