Well made
I haven't played the prequels but this seems quite good, you're clearly an experienced developer, and there are some nice innovations in the game. That said it still does seem a tad formulaic; as a defence game I guess this is somewhat inevitable. Still, it is enjoyable, but some adding some more layers to it might just well push across the boundary from good to excellent.
Maybe infusing it with more RTS type features, like placeable turrets, walls or somesuch could do this but also (and this would be really awesome): adding an attack mode, where you have to plan and conduct an invasion of some sort, rather than defending constantly. I think it could really work with the current engine you have, it has a lot going for it, and is a shame you have to use all this cool stuff to defend this portal thing all the time.
And one more thing: what is the music used ingame? I really like it, is it on NG?