Thanks very much for the comment. To add one more thing to what Peruman mentioned: If you have access to a XBox/game controller, I would personally recommend trying the game with the controller - I find that it's a different experience than playing with a mouse, easier/smoother to place the cursor on towers, easier to shoot (and because of that, I find that it makes it easier to play with more complex patterns where you can move from tower to tower and destroy enemies more quickly to stay on top of the invasion, rather than falling in to the trap pattern that you mentioned of just circling around one tower and and mostly dodging bullets as enemies pile up).
So, I don't have any sort of game controller (long live team keyboard!), so I won't be able to try that.
Being able to snipe towers across the map, yeah, that's a fair concern. So, lemme just up the difficulty in programming and say 'maybe the mouse should snap to the towers a bit' XD.
And yeah, running out of time in a game jam is a very typical thing, so no worries.