Thank you for the feedback. The towers do have an hp bar, but the insects do too much damage too fast, so you may not notice.
The ability of heal the towers is really good idea, and I think it could at least be applied on the upgrade, which would heal and increase the hp of the tower, or it may require specific button to repair it.
As for the score, the formula is "insects killed + current money * 2 + tower destroyed * 2", so I guess you got many towers destroyed and didn't have much money at the end, but indeed you got pretty far, thank you for playing!