With the high cost of upgrades, this game basically forces you to replay levels over and over to get points. Why do that? With so many created levels, the user should be encouraged to keep playing the new levels rather than be forced to keep doing the same thing over and over.
The expert bonus is a pretty useless motivator when its incredibly to get when you come back to it later when you are overpowered for the level. Which frankly is the only way to get the bonus because you're not strong enough on the first try.
When not going for high scores, the game is easy. You just put it on 3x speed and don't speed up the waves.
Also one of the keys too all tower defense games is good tower placement and good tower combinations. Putting in expert bonus and telling the player to spam the waves all at once takes away from the strategy of this because basically every tower is going to be shooting at once anyways, it matters less if you smartly put a tower at place it can shoot twice. And slow towers are basically useless in a situation like that.
The game does not put up a good challenge on its own and puts too much repetitiveness on the player. Two big flaws. I did love the upgrade system however. This game could have been much better if executed better.