Wait, it’s all AoT?
Attack on Titan (Shingeki no Kyojin) starts with humanity living within walls, fighting against giant humans — “titans” — to just survive. The anime is brutal, and any character can die — but it’s also very entertaining, and there are many layers of mystery to it.
The anime shifts its genres multiple times, but its initial premise, “humans fighting an external threat they can’t win against”, is not that unique. In fact, many anime are actually just Attack on Titan with different enemies if you think about it. Of course, the trope wasn't invented by it, but AoT influenced at least some of these series.
Kabaneri of the Iron Fortress
Okay, Kabaneri of the Iron Fortress (Koutetsujou no Kabaneri) is so similar to Attack on Titan that it was jokingly called “Attack on Zombie” by some. Both anime were made by Studio Wit, directed by the same person — it’s really hard to deny the similarities between these projects.
Unlike AoT, however, Kabaneri is an original project by the studio, not a manga adaptation. It concluded way earlier than AoT did, and while it included political elements, it didn’t go all-out with them like AoT did in its last arc.
Seraph of the End
Seraph of the End: Vampire Reign (Owari no Seraph) is Attack on Titan but with vampires… well, no, not really. The initial setup is similar, but even the tone of the anime is completely different — Seraph of the End is more focused on interpersonal drama.
That said, it does share enough similar elements to AoT, such as the whole military aesthetic, or the music, composed by Hiroyuki Sawano (who also worked on Kabaneri).
Neon Genesis Evangelion
No, Attack on Titan did not invent the “invading mysterious enemies and teenagers who fight them” trope. It wasn’t even particularly close.
Of course, Neon Genesis Evangelion didn’t, either — but this just shows how anything can be Attack on Titan if you bend the definition enough. In Evangelion, mankind fights against Angels, the true nature of which is only revealed way later.
A military unit, dedicated to slaying monsters who eat people. This is both Attack on Titan and Demon Slayer (Kimetsu no Yaiba), and both series are incredibly popular. The premise is easy to understand, and the anime themselves are hype and engaging — no wonder both served as an introduction to anime for many otaku.
Black Bullet
Funnily enough, after the human origins of titans in Attack on Titan were revealed, it became even more similar to Black Bullet. In the latter, the “Cursed Children” fight humans that have been turned into monsters by the Gastrea virus. It also deals with discrimination.
Unfortunately, the anime never had a proper ending — and the author retired from writing, so LNs won’t have one, either.
Knights of Sidonia
Knights of Sidonia (Sidonia no Kishi) takes the humanity versus unknown threat — in this case, aliens — premise and puts it into the far future. The anime is essentially a “zero to hero” journey for the protagonist, but there’s way more political agenda in play than there seems to be at first.
Don’t be afraid of the series being made in 3D CGI, by the way: it looks good, it just takes some time to get used to.
Terra Formars
Terra Formars is an anime about humans fighting giant monster cockroaches on Mars. Yes, it’s just as epic as it sounds like — and while the actual reason for fighting is way more complicated than it sounds at first, the anime is crazy enough to make it work.
It’s very brutal — perhaps even more brutal than AoT — and no cast member is safe.
Muv-Luv Alternative
The Muv-Luv Alternative visual novel served as one of the main inspirations for Attack on Titan. Humans fighting an external threat, military setting, a huge cast and a complex net of political issues.
The visual novel received an adaptation a few years ago — and while it was mostly panned (for poor animation and rushed pacing), the two spin-offs, Total Eclipse and Schwarzesmarken, are mostly watchable as standalones and aren’t as bad.
Garo
Garo: The Animation (Garo: Honoo no Kokuin) is actually just one of the installments in a huge franchise that is pretty popular in Japan. It includes anime, live action series and games.
The first anime series, however, is probably the most similar to Attack on Titan: the protagonist, driven by revenge for his mother, fights monsters that eat people. Garo is a more fantasy-themed version of that, though.
Kiss Dum: Engage Planet
Another take on the “humans versus aliens” setup, Kiss Dum: Engage Planet is a fairly unknown mecha series from the 2000s.
Despite that, it executes its premise fairly well, and is definitely worth watching if you are starving for anime where humanity has to fight to survive. The actual plot is also far more complex than it looks at first.