Not too bad, but graphics need work.
Well, this game has it's ups and downs for the most part.
The graphics are my main concern. The background was pretty poor, and would suggest taking more time to draw a better one, or simply get a good background off a website and use that. Also, the enemies as your turret as a whole were too big. Everything seems cramped in that small space you have to fire, so zooming out on the action by a large percent would help you see where the enemies come from better, and could also make the game harder, because there wold be more room to miss. The menu screen was also a bit plain for me. You had a large stage space to take up, but you really didn't take advantage of it. The game options seemed cramped in the corner. Some good things you did with the graphics was the intro for the most part, it was simple enough, but still had some nice style to it. I also like the way you made the cannon part of the turret, because it looked like metal.
The sounds of the game were the best part of it though. The music choices were also very nice to listen to, and being able to switch between them at will was good. Only one thing would be to add sound effects to so you can make everything sound more lively and real.
The gameplay of the game wasn't that bad either. The custom right click menu was a fine addition to the game, it made it more professional. The way the game works with you being able to pick whatever diffilculty you want at first bothers me. It might e better for some people who want more non-linear gameplay, but for a simple game like this, I would say that going for levels that increase in diffilculty would be a better choice. You could also tie the levels together with more of a plot, to make the game more engaging to play. Also, for endurance mode, there was no replay button after the game ends, so you might want to add that.
Overall, it's not the best game or anthing that really stands out, but it's still a solid game, that can provide some good diffilculty during gameplay.