This is a sequel done right. There's enough content here to provide new challenges to players who have mastered the original Defense Grid, but at the same time, this is still very much the same kind of game in all the right ways.
At first glance, the new content seems only modest compared to what is familiar. Almost all the old tower types and enemies are back, but the handful of new types of each and subtle rebalancing of the existing towers mean that new strategies need to be developed.
DG2 offers a plethora of choices as to how you play the game. From separate leaderboards that either allow or forbid juggling, difficulty levels from casual to very challenging, and even more game modes for each map than the first game, this takes what was until now the definitive tower defense game and makes it even better.
There have been some substantial changes to the economy and scoring system which, while I feel personally make for a more challenging game, do substantially alter the challenge curve of the basic level type. Gone is the compound interest earned from penny pinching. Instead, score is accrued independently from income. Efficiency is still rewarded, as you score more for killing each alien while you have less invested in towers. But the income from killing each enemy is unaffected, and additional income is a steady tick rather than being dependent on the amount you have banked. All this means that you can't just develop an optimised strategy for the early waves, and then find yourself with a huge bankroll. Levels can be genuinely challenging from beginning to end, and not just for people looking for pure score optimisation. I know this may actually be a bit of a disappointment for some, as the old game could sometimes be great for some uncomplicated unwinding at the end of a rough day. That is still there, but you'll have to dial the game back to easy in order to find that kind of experience.
I will say that I have yet to try the multiplayer modes, so I can't yet speak to that experience. But with that caveat aside, I wholeheartedly recommend DG2 to fans of the original and anyone even vaguely interested in the tower defence genre. If you're going to give these kinds of games a go, why not start off with the very best there is on offer?