A simple and basic strategy defense game. You start with a core building and build towers off of it. I don't think it's an amazing game but I don't think it deserves such a bad rating.
Some basic tips (use as needed; one strategy won't solve all levels):
* The plain structures are money-generating ones, so it's like an investment for more cash later on. If you make too many money towers, they'll get destroyed because you don't have enough early defense. Make too little, and you won't survive the later waves. Some levels require little/no money towers, other levels need lots.
* It's usually a bad idea to make tall towers because once a structure is taken down, everything further along is destroyed as well. Maybe there's a bonus but I didn't look into it.
* You can even let your core absorb hits to save your structures when you have the healing structure.
* The mine tower deals a lot of damage but limited range so some enemies will be able to attack from out of range. For example, in boss 1, if you make a horizontal line of mine towers and nothing above, the boss will stay in range of them.
* If your cash rate is high enough against certain enemies/bosses that instakill a structure, you can keep building your cheapest structure as fodder for every attack.
* You can build things underground and they're relatively safe but also don't protect your core.
I'm not good at real RTSs (like Starcraft, etc.) so I'm surprised a lot of reviewers had a hard time figuring out how to win. Once you figure out the towers and get the basic strategy down, most levels aren't as impossible as people think. If you're stuck on a level, you can just skip it: you can play any level of all the worlds immediately. I played my 1.5 hours and cleared most of the levels and all of the bosses. I don't care about score attack but that may appeal to some people for replayability. It may look pretty casual and the system is very basic but it can be a pretty challenging game that requires a certain level of strategy and luck to complete.