I enjoy Clash of Zombies for its incoherence. Clearly it has a zombie theme. It’s also science fiction — your base is a crashed spaceship which the upgrades are repairing. It also has comic book superheroes. If you’re looking for the eloquent way they combine, you can stop. Mechanics-wise, you might assume it’s a boring clone, but it tries some new things, including a new idea for monetizing.
As for the zombie theme, your 3rd resource is Survivors. The generator is a radio tower calling people fleeing from zombies to come towards your camp. That’s cool, but it means you’ll later be spending humans. In the mini-game where you defend your base, the computer attackers are zombies. You can capture some of them for 1-time use later. That’s the only way to use zombies in your attacks. Out of all the heroes, only 5 are zombies. Except 1 is really a mutant from Resident Evil, another is Stitches the blobby hulk from World of Warcraft, and the last is zombie president Trump. The cut-scene at the start shows a base being overrun with zombies and tells you to rescue some guy from it. I never saw anything more about that.
Sci-Fi-wise, the whole base is futuristic. Defenses are blasters and lasers. One troop have an energy blade and one is a robot. The dragon is a huge anti-grav pod.
Idea-wise, it’s a mixed bag. Some seem pointless. “Arena” mode has you attack one base it picks, for Arena points. They give weekly prizes. I don’t think that adds much to the game. You can attack a computer base where the HQ has about ten million health, using all your heroes but no troops, and the damage you inflict stays. There’s a second defense mode where you’re attacked by only heroes, not troops. Nothing wrong those things, but they’re not all that fun.
As for good ideas, defense mode allows you to assign builders to repair buildings under attack. It’s somewhat fun, it gives you something to do while you watch.
The defensive buildings have customizations, unique to each type: a stun chance, damage over time, power-up nearby defenses, and so on. That’s some nice extra effort. But they can only be bought with special currency which can only be bought with cash. That’s pretty much the worst thing to do in a Freemium game — a big, obvious “I bought this and am better than you”.
I suppose once you have superman fighting zombies on an alien planet in the year 2300, you’re not expecting the rest of the game to be a well-crafted masterpiece.