I've been a fan of the series since the original.
In a nutshell the game is hard to put in a nutshell. It's vaguely reminicesent of tower defense titles, but with an absolutely monstrous, game-defining twist. The enemy is not units, but rather Creeper-goop that covers the map and damages or destroys your structures on contact. The game is to fight back, and then destroy the goop spigots-"emitters"- or complete other objectives. The structures and enemies are all very simple and direct in their purpose and utility. The game itself however is extremely various in difficulty based on the map itself-size, features, and the strength of the emitters. There are additional enemies in addition to the emitters but they are not in all maps. The challenge of the game is all about defending yourself while you build your energy resources up before you push to destroy the emitters. Later, you can redo the maps to attempt to do so faster.
The game has a great custom map communtiy and a custom map generator after you finish the decently long and challenging campaign. Replayability is top notch.
I've gotten more fun and more hours out of this than a shameful number of AAA titles. All things considered, absolutely amazing for an indie.
I've had no issues with bugs, and the developer has been pretty good with support for those that do find them.
The only downside is there is no direct multiplayer of any sort. However, it really doesn't detract from all that you're already getting in this well-fleshed out title.
-There is no such thing as too much energy
-Stick to high ground when you can
-Don't land near emitters
-Don't build too many structures at once
-Don't botch your startup
-If you don't push, you can't win