It's a good improvement to the tower defense genre(forcing the player to have an objective during the waves as well). I do think the complete loss of towers is too harsh of an idea. If you really want to stick with tower destruction, then maybe give them a bit more HP(they go down way too fast). Also, once they are destroyed you can leave ruins there that can be repaired in the next turn for 1/4 of the original price. So it still punishes the player for not taking care of towers, but the game is more salvageable if a player messes up a wave.