Clash of Clans, a game by Supercell Entertainment, is very slightly similar to tower defense. You can buy defenses and upgrade them. However, it is completely multiplayer--the campaign is just there to start off new members.
You start extremely weak, of course. But it is fairly easy to get to a point where you will be able to attack other players successfully. You do this by training troops (barbarians are your standard fair for just swarming buildings, archers can attack from a distance and shoot over walls, goblins are used to get loot first, etc.) that you use by tapping or holding your finger on the screen where you want to attack from. Note the fact that you can attack and be attacked from all angles.
Later on, upgrades can take a while. But the times aren't ridiculous, as they are in some other games similar to this concept. I'd recommend spending as little as possible of your initial 500 gems. Gems are used to boost higher-level loot makers (gold mines and elixir collectors, and dark elixir drills when you get them), higher-level barracks (where you make your troops), and to finish anything being built instantly.
Oh, and yes, the higher-level walls get WAY overpriced. But then, you won't be worrying about that for a while.
Overall, I'd suggest this to anybody looking for a good-quality, long-term game. Especially if you like games with lots of multiplayer activity--that's pretty much the entire game's premise. Unfortunately, you will almost never find anything other than useless drivel being said in the global chat. This is where players from around the world can chat. I just ignore it, and use it only when I have to.
Lastly, you can join or create clans after you rebuild your clan castle (the grey, crumbling building). In clans, you can donate troops to one another, play with your friends (if you agree on a clan to meet in), and fight in clan wars. Clan wars are events that can be started by leaders and co-leaders of clans, in which two clans are pitted against one another. Only clans with 10 or more members can participate in clan wars. Players in a clan war have two attacks each, and the clan with the most stars (basically a way of telling how well you attacked) by the end wins.
So would I suggest this game? If you like multiplayer and don't mind some minor problems, absolutely.