Onslaught2 (which does not seem to be related to another game by the name of Onslaught on the same site-apparently the "2" is just to distinguish it in searches and such) is a fun tower defense game with depth that isn't immediately obvious.
When I say "depth that isn't immediately obvious," I mean "either you must experiment endlessly to figure out what's what, or look up stuff on various web pages and YouTube." The in-game tutorial level is simply abysmal, both in information it supplies and in explaining things beyond telling you what to find.
When I first played it, I looked at the four basic turrets (cannon, laser, rocket, and tazer-and yes, taser is misspelled even though the common misspelling of "lazer" isn't present). After some initial examination, I decided "well, the laser is awesome, but I can't figure out why anybody would use the other ones."
Why would I use lasers in the maniacal fashion shown here?
Because before upgrading turrets, lasers are the most obviously cool and nifty turrets in the game. When multiple lasers are close enough to each other, some of them will use their own fire to power up the others, allowing a map crammed full of lasers to bring most of its power to bear on a single target.
Unfortunately, even with extensive upgrades, the endlessly stronger waves of the game will wipe you out somewhere before the hundredth wave. (Incidentally, waves in this game are linearly stronger-near as I can tell, their hitpoints and the amount of money you get from killing them progresses in a simple whole-number progression, corresponding to their wave number. So the first enemies give you a dollar for killing them, the second wave's give you two, and so forth, though you can't see their exact hitpoints but instead the percentage of their health. On the subject of enemies, it's clear that the game's creator wasn't too concerned with realism or anything, as the enemies are disparate unanimated objects such as motorcycle helmets, masks, cockroaches, octopodes, printers, eyeballs, spaceships, and little stars with faces on them. There are minor variations between them-some are slower and tougher while others are faster and weaker-but they aren't important.)
So I couldn't figure out how things were supposed to get better. Sure, you could try other turrets, but the damage potential just doesn't seem to be there. In fact, the game claims the others have special abilities the same way that the lasers do, and I couldn't figure out what they were actually supposed to be doing, because they never seemed to go off. And what are these "combos" that the guide claims you can use, anyway?
After slogging through the tutorial (which isn't any fun), I found out that upgrading turrets was necessary to gain access to any of their special abilities besides that of the laser's. This is not a good, intuitive way to put a game together, guys.
If you upgrade a cannon or a tazer in a certain way, it will gain the ability to "frenzy," which means that periodically, its firing rate will spike sharply and it will be able to deal a lot more damage for a few moments (and look like they're having turret seizures). Happily, they do this often enough that you can actually set them up so that if you have enough of them and time their upgrades properly, there will nearly always be an active "frenzy" going.
A properly upgraded rockets will launch missiles that, when there are no targets in range, will go into a "holding pattern" and wait until there's something available to attack. In the right circumstances, this can lead to dozens of rockets hovering in mid-air at a time and wiping out the first few guys in a wave.
So yeah, much better than first impressions.
It gets better: If you put turrets with fully upgraded damage next to each other, they will periodically fire off "combos," special weapons or attacks that are basically the main point of and most powerful thing within the game.
So the game is fun, but only if you can figure out and learn all this stuff, with either guides from other sites or by trial and error.
There's actually a bit more. There are numerous "utility" turrets that amp up your other turrets and must be unlocked after a certain number of waves/kills, but I can't bring myself to use them because they're boring. There are also three massively expensive "super turrets" you can unlock, and a special turret called the "combonly" that magically absorbs other turrets nearby and "learns" an attack based on what combo those turrets perform in the correct circumstances. (That makes no sense without context, but it's actually a fair bit more intuitive than it sounds, once you've seen somebody else do it. For information on this kind of thing, go to this site.)
The purple turrets in the above screenshot are a set of combonlies that have been set to the powerful "black hole" combo, which is the only attack in the game that doesn't score kills or make money because it wipes things out regardless of hitpoints. If it did score kills, it'd be pretty simple to play the game forever once you had enough of them set up. (In a rather neat touch, the black holes not only eat all enemies in their reach, they also gobble up weapons fire and mines. Those little gray circles you can see on the path in some of the screenshots are mines, which are among the combos you need multiple cannons for.)
If the game isn't progressing quickly enough for you, there's also the fairly standard "send next wave" feature. I actually have lost more often by overzealously clicking or accidentally double-clicking this button than probably any other way, because there's no limit on how often you can push that button. Case in point:
This technically wasn't actually one of the times I lost... because the game crashed Flash Player within seconds of taking that screenshot because of the sheer volume of enemies.
In fact, Flash crashed so badly that when I refreshed the page, it didn't get better. So I restarted Firefox.
It still wasn't working.
I had to restart the whole computer to fix Flash Player.
Anyway, there are some bad things, too. First, the interface is straight-up awful. The basic menu that you use to select your turrets can only show one at a time and must be clicked through one at a time, and so you have to use another dialogue box in the game which is only marginally better (well, it's much better for browsing through turrets, but it's awful because it gets in the way and is generally obnoxious unless you minimize it). You're supposed to be able to move turrets, but even with the online guides, I still haven't figured out how to do so. And as noted, the game's balance and design is basically completely unintuitive.
The game also lacks music, but I reckon you can take care of that yourself with a YouTube window or a playlist or an iPod or whatever.
So in sum, a fun game with significant problems that can mostly be worked around if you're willing to look things up and learn stuff. If you have a fair bit of energy to invest in it, this is a good tower defense game, but if not, you should probably avoid it.