As I understand it now...

From looking at a number of boards and seeing what they did, if you wanted to call it a homage or "based on" you might be able to pull it off. Apparently this take is a post apocalyptic world in the year 825 (?) and the defenders are in Japan in the last walled city holding out against the titans. Oh yes, they can use weapons like RPGs now too! I think the best laugh I got was from a poster on another board who had this to say about the thinking process that went into the making of the film:

"PraetorBrian Ashcraft
4/22/15 9:37am
I feel like the thought process was probably like this:

Hmm, let’s make a movie that will make lots of money. We’re pretty conservative, so it also has to be a sure thing. That means we have to make a movie based on something that is currently really popular, and then spend only as much as is absolutely necessary to ensure a profit. What’s really popular right now?
Attack on Titan.
Okay, let’s do that. Tell me about it.
It’s a manga/anime, set in early 19th century Europe...
Great, let’s base it in modern Japan. We’ll only use Asian actors too.
...well, um, hmm. Anyway, humanity has been reduced to a single huge city with 3 concentric ring walls around it. All their farming and food production and industry and everything is contained within those walls.
Great, we’re in Tokyo, we’ll base it in Tokyo.
There’s not a lot of farmland in Tokyo...
CG.
Okay.
So what’s the conflict? Where’s the drama!?
Well, these huge, non-communicative humanoid things called titans try to eat the humans, hence the walls, but the brave, adventurous, cavalier defenders of the city in the Survey Corp regularly venture outside the walls to learn more about the titans and how to defeat them. Again, this is based roughly on early 19th century Europe, so they ride horses and carry swords. The city walls are manned with cannons.
Great, let’s toss some RPGs and Stinger missiles in there, it’ll be great!"

O-K-A-Y...