It's using Adobe Air for Steam integration and Flash as engine. But the game is pretty solid as long as you don't push too far into the insane end-game where hit points are beyond trillions .
Trillions, pssh.Try scientific notation on for size
Actually that screenshot doesn't show how much health or damage things had. But I remember that the forward gun* had a per-shot damage listing in the neighborhood of 4×1018
I don't hold the highscore any more, as the game updated breaking things into difficulty levels, which reset all the scores. And I haven't bothered re-achieving it.** Then again, no one else has either, due to the sheer tedium (I was not the first person to discover this particular cheese, it was/is a long-standing tactic, I was just sadistic about placement in order to maximize the result).
But yeah, same response as to end game Gemcraft: mind-numbingly boring due to the level of perfection required.
*For those not familiar with Onslaught 2, the laser guns would chain. When a laser gun fired, any laser gun anywhere on the field that had a shot ready and had the firee within its range would fire into the firee and add its damage * 1.25, and then every laser gun within range of those would fire... So a chain of four would do 576% the damage of 1 tower. Check the screenshot, look at how long that chain is. 149 turrets, that if the only enemy is at the very start of the path, each tower is capable of being boosted by only 1 other tower. No splits, maximizing that exponential. The other towers there are all damage/range exchangers. +300% damage -66% range.
**The game took over 6 hours, for most of which I was AFK (once I'd pixel-perfect placed all the towers using Windows accessibility options and Flash's zoom feature and got everything upgraded just right I could walk away).