Demons vs Fairyland is one of the outliers in the family of "build some garrisons as well as towers" tower defense games.
Unlike other similar titles, you don't have build points to build at; instead, you build larger structures with no offensive abilities of their own, and then can build actual towers and garrisons around them.
Incidentally, you're the demons in this game, and you're kidnapping the fairy children. The "attackers" are fairies who are trying to rescue the children from you. And for some reason they laugh like psycho jerks when they manage to get ahold of a kid. (When I was explaining aspects of this game to her, my sister protested that fairies don't rescue children, they steal them; then I explained that they were the fairies' own children, which caused her to protest that they don't protect their own children, they give them away. I suggested perhaps they cared about demons specifically stealing their children, and she said, "Okay, well maybe.")
There are a lot of things to like about this game, such as the charming graphics and the absolutely amazing upgrade of a "fake kid" that you can purchase, which distracts the attackers/rescuers and consumes their lifeforce when they try to "rescue" it. You can also build multiple of the large "support" structures to amplify the structures they enable, which is a neat twist and something I really enjoy exploiting. (The garrison enabler causes an extra soldier to come out of each garrison it's supporting, so I like to cram three or four around my garrisons and upgrade them so that each garrison spits out about eight guys instead of the normal two.)
And then the game is crammed full of senseless things that frustrate me.
Chief among them is that there's an unclear and arbitrary feature that keeps you from putting down the support structures in places you think you should be able to. This is one of my two least favorite things in any RTS/tower defense games.
The other big thing (which is my other least favorite thing in any RTS/tower defense game) is that it's got the ramping cost system on the towers, where the price goes up on a tower the more of them you own; while this particular game probably needs a version of that for balance because of the aforementioned support system, this is the only game I've ever played where the price on structures triples instead of doubles.
These wouldn't be as much of a problem if every map in the game was shaped like this one.
This map is simple to play, because it gives you an excellent place to mass your splash damage towers and your garrisons. (There's a tower that fires single-target projectiles, but sadly I find it a bit useless. Also, it's a tiny archery tower with a generic demon in it, nowhere near as adorable as a "haunted house" that spits out skeletons as soldiers or what looks like a little hot tub with a tiny cartoon Cthulhu sitting in it that fires pixel explosions of death.)
But way too many make it nearly impossible to concentrate your firepower and garrisons in the same place, and it's one of those games where that's super-necessary.
There is a perk of sorts, that being that you gain a small amount of experience for upgrades even if you've played a stage through before, but that's small comfort when you could be playing a superior game.
Anyway, while this game isn't great, it can be engaging for a little while, and the novelty of the structure tree is worth a look. I can say that this game is worth playing a bit if you enjoy tower defense games, especially the ones in its subgenre.