What do you get when you cross Missile Command with tower defense? Hopefully, a better game than PlanetDefender. PlanetDefender tries to vary the tried-and-true tower defense formula somewhat, but unfortunately the game that results is somewhat less interesting than the sum of its parts.
Like a tower defense game, you face a number of waves of malignant, increasingly powerful enemy ships bent on wreaking destruction on your peaceful planet, and you have to build various things to blast them out of the sky before they can do too much damage. However, unlike your typical tower defense game, the enemy ships can and will shoot back at you. They can either damage (and possibly even destroy) your defensive structures, or kill your population, which is bad news, because your population is your tax base. Unlike your typical tower defense game, where you get money for every enemy killed, here you get money over time based on your current population. In turn, you can build the usual array of weaponry, economic centers (which increase the money gain from your population), or bunkers to protect your population, as well as research some additional technologies.
Now, on to the flaws of the game. First of all, there's no strategy (at least none that I can see) to placing your units, unlike in a typical tower defense game. You just place them somewhere on the planet and they shoot at the enemy. This takes out one rather large chunk of strategy. Secondly, the pacing of the game is terrible. At the beginning of the game, you end up waiting for very long periods of time for enough money to accumulate in order to buy the next thing you want to buy. (This is, to a greater or lesser extent, an issue in any tower defense game, but PlanetDefender does not do a good job regulating the pace.) At the end of the game, when everything is maximally upgraded, you also just end up sitting around twiddling your thumbs while watching the alien fleets bravely float into the meat grinder. Thirdly, the interface is really annoying -- when your mouse isn't over the planet, all of your buildings fade away, presumably so you can see -- well, it's not actually clear what the purpose of this is. I suppose you can enjoy the majestic beauty of Earth's oceans and clouds, but this feature doesn't bring any useful information to your fingertips; in fact, it takes it away.
But the most unforgivable sin is very simple. As I mentioned earlier, a key to building a successful tower defense game is feedback -- the player needs to be able to understand what's working and what's not working, so he can adjust his strategy accordingly. PlanetDefender provides almost none of that -- you can't look at the enemy ships' health bars, so you have no idea how close you may or may not be to destroying them. You also can't see where on the planet you're taking the most damage, which might be useful if you were thinking of, say, placing one of those fortified bunkers. The only thing you can see is if your defensive structures are taking damage, since there's a little bar which gradually increases (not that it's at all clear to the novice player what that bar is). Even here, though, when a structure gets destroyed, there's no notification, so if your notice happens to be elsewhere on the battlefield, you can be blissfully unaware that a key component of your defenses has suddenly disappeared (though if this happens, you're screwed anyway, in all likelihood).
PlanetDefender also exhibits the hump in difficulty common to tower defense games. That is, since your income is dependent on your population, if you make a mistake which causes more of your population to die, then you have less income, which means you'll be able to build fewer defensive structures, which means more of your population to die, etc. That is, there's a strong positive feedback. As a result, small improvements in skill can result in large differences in the outcome. Like I said, this is common to many tower defense games (especially those with interest), but it seems to be especially problematic in PlanetDefender.
The graphics are decent, although small; the music gets boring fast, and the sound effects are nothing special. Overall, I was very glad to finish this game and get the badge, and was annoyed that it took as long as it did.