Original idea, but...
The first thing I noticed was that the game was like.. Xeno Tactic. The map, at least, and at first, I enjoyed it, because that meant that I could apply the exact same tactics, and since I loved Xeno Tactics... I thought I would do the same for this game.
The idea was original. The game itself? No.
The game was not complicated, it was not hard. It was actually kind of boring.
Whereas in Xeno Tactic, there was a lot more variety of weapons, towers, all that would happen when you upgraded was... an increase in damage. Nothing in range, attack rate, no outside benefits, and basically what the game meant was: Defence high at the beginning for hijakers, maze like mad, and when you're used to where the viruses fly over, pump that spot full of virus protection. It was dull.
If you want to make a successful game, make one that's original. You have the idea. Now do something NEW with it. Don't COPY defences, if you do that, we're just going to play the game that you copied it from. Make us want to play YOUR game, not another game.
It was a good try, that's what I have to say.
It might seem a bit harsh to give you a 4, especially since everyone knows that I am a hardcore lover of defence games. But I can't help but to tell you that there are better defence games out there, and you've brought nothing new to the table.
Good try though.
My main suggestion is as stated above: you need to find something new. Hard, I know.
Smaller, easier suggestions.
Upgrades: make it do something other than just... damage?
Change of pace: I can see that you tried to speed up the game by adding a timer, but seriously? There's no point in doing that if everything shoots at the same rate. Not only are we bored by the animation and the rate of fire, but I was annoyed at how they kept spawning, and there was nothing that I could do, since the towers shot so damn slow.
I can't really give you much constructive criticism, other than to stop taking pieces of defence game that you've seen from here and there, and to make something new. Honestly.