This review will probably get me lynched but it is just my opinion here.
Another world after an apocalypis tale with a young hero. Starts with years of peace behind 50 meter tall walls. When a giant blows down that wall the world changes again.
A very popular anime but one I am not sure why is so popular. The lack of back story or explanation in the first few episodes makes you wonder WTF. Just not an anime that grabbed my attention.