Thanks for playing! Yes, spamming -5s and -10s is one way to do it. But you can win with a total of 7 towers.
I don't want to spoil the game for other players, but essentially it's all about when and how you place them, considering they will always aim for the numbers that are closer to your base.
Towers have a cooldown between shots, so that's why numbers only take certain damage while they are in range. This cooldown is currently not indicated visually and I might add it in the future once the voting and jam are over.
Again thanks for playing and taking the time to comment! :D