Some points to improve on.
A fun game with great visuals, great sound effects (basically copied from the real game itself) and a great concept, but I have a few suggestions that might help to improve this game.
I did notice a little 'problem' with the game whilst playing that made it ridicuously easy.... I found that if I stood with my back to my sentry, whilst standing in front of my dispenser, I could fire into the enemy base without stopping, and from the other end of the map. Any spies who came along would would sap the dispenser and then get blasted by the sentry. All in all, it was an invincible defence, and it kinda made the game a little dull.
To remedy this, instead of the map being just being one long, continuous flat plane, you could add multiple ledges to the map. This would make it difficult to fire right across the map from one end to the other. Secondly, you could make the shotgun's power decrease with range, or limit the shotgun's range to what is visible on screen.
If you were to make a sequel to this game, I would suggest a top-down "bird's eye view" camera, since side-scroller action tower defence-style games are rather bland, on the whole.
A final point - In a real game of TF2, you can hit spies whilst they are cloaked.