It's hard to be too rough on Horn Swaggle Islands at 400 points when Crystal Defenders is lurking on the Arcade section for 800 points. I thought Crystal Defenders was terrible, while Horn Swaggle Islands is merely okay. The maps look nice, the creeps have cute voices (reminiscent of the original Worms games), and there is some depth to the gameplay. I had two major problems with Horn Swaggle Islands, though: slowness and polish.
Even Crystal Defenders has a toggle that lets you speed up the gameplay. I beat the first two maps, and factoring in a couple of deaths along the way it took well over two hours. It may've been because it was three or four in the afternoon, but during the second map I found myself nodding out a couple of times. You can initiate a wave "early," but you can't start the next wave until the wave before it is completely "dispensed." During the second map (when I had the hang of things), nothing ever came close to making it once I had my upgraded crow's nest (sniper) towers in place... meaning most of the gameplay involved just sitting and waiting for the next wave to start (instead of perhaps sending a couple of waves at once to make it go faster).
The controls bugged me at first (a mouse-like cursor, but constrained to a grid), but after a couple of games it actually felt pretty responsive. The information screens are a pain in the ass to navigate, and they seem laid out by someone who's never played a tower defense game before. The current stats are shown on a different screen from what the next upgrade will be, which means if you're trying to decide whether or not the upgrade is worth it you'll be forced to constantly shuffle between two screen. Several of the screens have the wrong information on them altogether, which means you have to memorize how much things actually cost. For example, the cannons are listed at 8 gold but actually cost 12. The crow's nests are listed at 16 but actually cost 20--though at least the upgrade prices seemed accurate. This sort of mismatch is indicative that there was some last-minute tweaking going on, and the developer probably should have spent a little more time playtesting.
When you can't afford an item, it places $1 barrels in the shape of the item. This is meant to act as a placeholder, but really it only serves to confuse and frustrate. Similarly, you've got to abandon some preconceived notions of tower defense strategy or you'll get your ass handed to you. The first few times I played, I used a standard DTD strategy of building a maze of level one cannons. I got my ass kicked. A lot. Eventually I realized that the $1 barrels are the most important piece in Horn Swaggle Islands--you're much better off building a couple of units and upgrading the heck out of them.
A couple of other features feel tacked on: the "X marks the spot" bits and the ship's hold to rearrange pieces (I never bothered with either). All in all, Horn Swaggle Island does "feel" like a real tower defense game. Because of the slowness of gameplay, I wouldn't recommend it to anyone but a truly hardcore strategy fan... but it's not bad. I don't think it's worth the 400 point price tag, but it's definitely the best tower defense game I've played so far on the XBox (looking at you, Crystal Defenders).