Defend your base.

I played for about 1 and a half hour, for a longest survival time of 50:19. It was actually easy the second time I tried, as snipers are really powerful. They aren't completely overpowered however, as they are easily killed if left undefended. The units available to the player were well balanced with each other, while being maybe a little too powerful for the enemies. The tank seemed useless, however.

While there was a plot, it wasn't really important for the gameplay. You should integrate it a little better. As it is, skipping it didn't change anything, it just looked like another defense game, like so many others. (Flash games) It's not completely generic, and the idea is well developed, but you need a plot inside the gameplay, not just outside. Boss battles, reinforcements, onslaughts, heroes, etc. could be that little extra.
I would also add more levels, with different base designs. The defenders moved too slowly. Maybe make them move faster inside the base.