I've been playing (and reading) some while building up enthusiasm for my next project, whatever that will be. I happened to find a great tower defense game via Twitter, and when I started recommending it in turn people asked me, "are you planning to make one of these?"

My first reaction to that was, "neah, there are too many in the genre as it is". Then, "you know, I haven't played any in a long while". So I set out to look for more, and promptly found another gem. Which wasn't very hard, as apparently there are only two kinds of tower defense games: excellent and terrible, with nothing in-between.

No, I'm not planning to make one myself. But if I was, what lessons could I learn from Runestone Defense and Pirates Love Daisies?

First of all, a good game of this kind must be relatively simple. When wave upon wave of enemies is raining down on you, the last thing you want is having to worry whether this tower needs a better rate of fire or more damage. Just give me a general upgrade option and be done with it! I have enough trouble choosing between an upgrade and a new tower in the first place. Luckily, both of the games I'm discussing here get it right: a handful of defender types, a handful of enemies and that's that. Where they differ is in the balance: in Runestone upgrade costs and new tower costs are comparable, yet the author advises outright that upgrades are more effective (though you'll want to interleave them for both tactical and cost reasons). In Pirates a new pirate is much cheaper than an upgrade, yet he's hardly useless.

Another lesson is that style matters. Much of the fun in Runestone is watching the undead crumble into a pile of bones with a satisfying "splurch" noise. In Pirates it's watching the eponymous fellows unleash an epic fusillade that mows down the invading critters like nobody's business. It may be possible to make things fun enough with vector graphics and electronic bleeps, but I'm yet to see it done.

Then there's the issue of level design. Whether you're building your own maze out of towers or placing your defenders around a fixed path, you'll want variety and surprises. In Runestone the skeletons are adept at avoiding your towers, and mixed enemy waves can easily trick a simplistic defense system. In Pirates you have rats taking narrow paths no other critter can, but also seagulls that ignore the paths entirely. Most other games just have nearly identical enemies filing in a straight line to overwhelm you with no recourse, until you figure out the trick and turn the level into a meat grinder. Boring!

That doesn't mean the two games are perfect. Half the tower types in Runestone Defense are useless, and I was unable to figure out level 7 or 8 — the one with the two large triangular areas. Also, one mistake tends to mean instant defeat. Pirates Love Daisies is too easy on "easy" and probably too hard on higher difficulty. (I only lost the first time I played the game... but I never finished a level with all my daisies). It also plays music in a loop all the time, and you can only turn sound off entirely.

But these are quibbles. In a genre many perceive as highly formulaic, there is obviously plenty of room to get things both very right and very wrong, and I just found two great sources of inspiration. You know, just in case.