In order to spoil as little as possible, I'll note that Attack on Titan takes a very common trope (humanity huddled together in its last refuge, beset by some terrible force) and refreshes it by making our collective nemesis... person-eating giants. Lots of them. Ranging from 12 feet to 50 feet in height. They can be hurt, but they regenerate faster than Wolverine: the only reliable kill-shot comes by targeting a small spot at the back of the neck.
The art depicts the giants as fantastic grotesques with idiotic expressions on their faces (mostly) and figures somewhat distorted from the human norm, enough so and varied enough that different monsters always seem to bring with them a horrific shock of the new. They're truly disturbing visual creations -- they create that frisson of un-ease that one seeks in horror but rarely finds. That they're beautifully integrated into vertiginous, sweeping battle sequences is also a triumph, a thrilling combination of horror and action.
Where did they come from? What are they? Why do they eat only humans? Why, having eaten pretty much everyone on the planet a century earlier, are they still alive and apparently non-starved? Where on Earth is the Last Redoubt of humanity (thanks, William Hope Hodgson!)? What secrets does one of our young protagonists have because of his vanished father's scientific enquiries into the origins of the Titans, secrets somehow hidden in his memory but unavailable to him?
Well, read the series. It's a blast. There are many adult characters, but the main protagonists are all in their late teens, new to the job of giant-killing. The writing is sharp, the characterization somewhat stereotypical. But when it comes to the giants and the battle sequences, Attack on Titan is terrific, horrific fun. And as it will clock in at about 4000 pages whenever it finishes, there's a lot more where these volumes came from. Highly recommended.