Archeo Tech is a common situation in anime, where a given community owns some artifacts much more advanced than its average technological level. I'm sure there are many cases, but the most notable offenders I have in mind are Nausicaa of the Valley of the Wind and Vision of Escaflowne.
In both we find communities with an average technological level not much above steel swords and chain mail, but Nausicaa uses an individual flying wing to travel around, and the Fanelians own self-powered bipedal combat exoskeletons (mechas).
As far as I recall, Nausicaa gives no explanation for the mismatched technologies, while I've watched only a small part of Escaflowne; from what I gather, tho, the Escaflowne - a most powerful combat suit powered by crystals extracted from dragons - was built by an ancient race.
Another curiosity is that, with rare exceptions (Mononoke Hime) in these alternate universes and timelines nobody has ever developed firearms. But this seems to be a common shortcoming
Archeo Tech isn't really bad in itself, but if not handled well can spoil a show for those with engineer's disease: in our neverending quest to discover how things work we need at least plausible explanations for why the princess of a city-state of medieval farmers can zoom around on a jet-powered flying machine (and where does she get the fuel? Who does the maintenance? You get the idea...). To give you an idea of the level of mismatch, it's like the Taliban owning and employing perfectly efficient B-2 Spirit bombers.
Of course, the story in Nausicaa has a very different focus, so a small incongruence in machinery is no big deal. But in Escaflowne the mechas are a main plot element and therefore contain a much higher screwup potential; I hope their story is developed properly. Because the other elements of that anime haven't really impressed me so far.
There is another egregious case of Archeo Tech - or better, of reverse archeo tech - in The Matrix Revolutions. The human rebels of Zion live in a place which looks a lot like a early-1900 factory stripped of its machinery, while they have very powerful computers and ships with a futuristic (and unexplained) propulsion system. But most notably, their main defensive gear is composed of more self-powered combat exoskeletons which have a distinct vintage look. No, they don't look old and worn, an neither rough for being built in haste with improper tooling. They look like they're made of state-of-the-art Victorian steel beams. But they don't have armor for the pilot. Seriously guys: you went through the pain of manufacturing the whole thing in the first place, fitting a couple of machineguns onto it... and you forgot or couldn't be bothered to add some armor (and communication and targeting systems, but let's not be picky here) for the pilot of a machine designed for fighting?? This is just too silly; inexcusable, even.