hey!!! good attempt
i say good attempt because i see where the idea would've immersed from.
but didn't quite play as fast, sorta like 30 fps compared to 40. I wish it was more fast paced. I also believe that letting the cursor slide to the _xmouse and the _ymouse sorta added on to this dilemma. and the minor glitches like, the Mouse.Show was never used for returning to the main menu, and when i went to look at the instruction(after playing and dieing) my mouse was nowhere to be seen as well as the custom cursor u created. the music also was never stoped at the return to the menu. all being minor easy thing to fix(i'm guessing)
generally not my type of game, as i would expect the space invaders theme would be old now(only example that came to my thoughts).
also a idea, not that you need to take any of this advice, but perhaps having the cannon or turret being on the same field would make the multi-tasking more enjoyable. a decent game that i expect is more of a demo. but if you could take this and give it a story, and more fast paced action, a moving background(perhaps a space theme)and you got yourself a great game!!!