So what's this game about? McDroid is an action tower defense game. Your ultimate goal is to defend your shuttle against waves of enemies. To aid you, you can build various types of towers. The action part comes in with McDroid's ability to take up to three towers and carry them on his back.
What makes it unique? Two things: (1) The interplay between your towers and the environment and (2) the high difficulty.
To build your towers, you need to collect strawberries. You get strawberries by planting glowing green seeds (which come from enemies) in specific dirt patches on the ground. The strawberries will sprout and then you must collect them and return to a dropoff point. The strategy comes in that these dropoff points are often not nearby. You can build towers which serve as dropoff points (and even collect strawberries for you), but you'll then need to defend them. The decision-making in determining what resources to collect and when adds a bit to the game. It's further compounded by bliss engines, which are a specific tower type that prevents corruption on the ground (which comes from enemies and natural disasters) and boosts your towers and strawberry growth. Don't get me wrong, you don't need to be a master strategist to get the most out of your resources, but it does make each level a bit different and it's far better than the typical "kill enemies, get $" common amongst other TDs. In this regard, it plays a bit more like an RTS.
The difficulty is the star of the show. The overall rating for this game is not great. I'd imagine at least part of that comes from the fact that this game is really, really tough. Even the normal difficulty stuff cannot be done without care. You need to decide what type of towers to buy, which ones to upgrade, and where to place them. It's simple, yet very well-designed.
Perhaps even more interesting is the way the difficulty levels in the game are designed. Once you beat normal, you'll unlock challenge. This is the same arena as last time, but with harder enemies. The catch is that you begin challenge exactly where you left off on normal. If you squandered your strawberries and left half your towers destroyed at the end of a mission, you'll find yourself radically outgunned in challenge. Beyond challenge, there's a third nightmare difficulty. Beating nightmare requires you to thoughtfully manage your towers throughout all three difficulty levels, which is an extremely cool concept I hope more TDs adopt in the future. You can, of course, go back to an earlier difficulty and right your wrongs.
Achievements? Not as hard as the ratio makes them out to be. If you like the game, you'll be able to get them all no problem. It'll take some work, but no more than 20-30 hours if you're skilled.
Should you buy this? Absolutely. It's dirt cheap and it's the best action TD I've ever played, and I say that as a fan of the genre. For reference, my favorite TD on Xbox is Defense Technica and my favorite action TD is Sanctum 2. If you're a fan of the genre, I'd wager you'll love this. If you like high challenge, you'll probably like it as well. If you prefer to relax and play games casually, you'll likely find this game simply asks too much of you, and that's ok. Just know what you're getting into.