This game has all the trappings of the tried and true tower defense formula:
There's the paths, baddies walk along it. There's the Land / Air dichotomy. There's the cheap damaeg tower, the slowing tower, the damage over time tower, the expensive zapping tower. There's some abilities you can use when you need that extra ooomph against the baddies when you need it. There's the mechanic of gaining interest on the money you've saved. There's occasional baddies that are resistant to X. There's the standard list of enemies: The Standard Baddie, The Swarm, The Slow Tough Guy.
So far, it has all the trappings of a standard TD game.
The feature that makes Sentinel 3 different than most tower defense games is your Sentinel, a robotic character that can jump back and forth between gates on the map and engages in combat on its own. It has its own set of abilities and can be leveled up over time.
Unfortunately, the Sentinel isn't engaging or interesting enough to raise this game from generic to something special. This is especially true in the later waves of the later maps: The Sentinel's capabilities are of little importance compared to your towers as the game progresses.
Also, there's very little in the way of interesting choices.
There's a limited number of slots for towers available to you, but you can purchase more over time. Even if the tower selection were more limited, it's possible to beat the campaign and get quite far in the Endless modes with just 3 different towers. Tower placement, in general, isn't all that critical either. Stick the slowing towers in the right spot, spam damage everywhere else.
Although the different levels of difficulty do indeed make the game more difficult, they don't make the game more interesting or encourage you to make different choices.
The only real choice one has in the game is where to spend the upgrade points on your Sentinel; you don't have enough to completely max out all of the stats. Unfortunately, as mentioned above, the Sentinel is almost inconsequential later on, so your choices here don't matter all that much past the first 20 waves or so.
If you're a Tower Defense Junkiie and absolutely must play everything available to you then you might want to purchase this. For everyone else you may want to pass, despite the low price of the game itself. Better, more engaging experiences can be found on many Flash game sites for free.