It's possible, if not probable, that the door itself was made and installed by the gods before the Amazons were chosen to guard it
Actually not - there's a story in the first or second Annual about the first post-Exile amazon death, which involves them actually building the door. Otherwise, that would be the beginning of a decent out, but as-is, the Door remains a major contradiction.
Thing is, I could be made relatively happy with minimal technological advancement, for the very reasons you mention - energy is always an issue, and you really can't jump straight to hydro or solar from zero... at least not without magic. Which would have been the other acceptable solution - magitech, where neither the magic nor the tech are super-incredible-sophisticated, but in combination make their society at least our equal in power. Who needs guns when you can mass-manufacture +1 shocking crossbows that fire missiles at the speed of light? I'd prefer straight technology just because it's harder to dismiss and it's a good shorthand for "seriously, these guys are advanced, they're better than us," but a mix would be plausible as well and perhaps better suited to distinguishing them culturally and flavor-wise.
And hey, a greater emphasis on magic in her culture might help people see how fucking ridiculous Bruce/Diana is. Seriously. It's like some WWII soldier who can't let a single fight pass without muttering about how much he hates germans trying to date some chick from Berlin.