More of a combination between Arkanoid and Galaga, but I get the point.
The basics: You need a pause button, as others have said, and a break between waves where you can buy upgrades.
Other than that, there's just WAY too much going on, and not nearly enough information. Considering you have to watch the screen constantly for the incoming ball and enemies, it took me a while to figure out all my money was going to archers. There's absolutely no indication that they cost money after building the barracks. Plus, there's no indication that your archers are DOING anything, except once in a while an enemy bloops out.
Also, guiding the paddle, click-killing enemies, and buying upgrades was just WAY too much to be tied to the mouse. You're constantly trying to do 3 things at once, and when things really start heating up it's impossible to do all (or any) of them efficiently.
This game gets high marks for ambition. It's a good concept and really addictive, but after a while it just gets frustrating, and has an extreme chance of inciting a nervous breakdown.
Also, on a personal note, is there some law somewhere that says all castle defense games must include archers, wizards, repairs, wall upgrades, and that's it? Where's the innovation? I've played a few Castle Attack games where the castle has a weapon that drops oil/water/cannonballs on frontline attackers, but never played a defense game with the same upgrade. Just one of my pet peeves. No formula is perfect, you can always do something to improve a game. This one made a step in the right direction with the ball/paddle concept, but it wasn't implemented as well as it could have been.
Not a bad game, but it could have been much better.