I really liked the protecting the carriage idea for a game like this. The game has potential to be pretty fun if you added some unique mechanics. Maybe the path the carriage is taking has some importance, or the carriage breaks down and needs to be fixed every now and then. Maybe instead of scoring the player by how many seconds they managed to survive, the score could be the distance the carriage has managed to go. I think building the game more around protecting a MOVING carriage instead of the gameplay being basically the same as just defending a tower could be unique.