They don't make 'em like this anymore- and that's a good thing.
This was a pretty standard science fantasy/pulp romp, complete with Atlantis, time travel, parallel dimensions, and swordsmen. Lots and lots of blonde swordsmen.
The trouble is that the execution of this is ... pretty racist. All the bad guys are brown-skinned, dark-haired, oily and hook-nosed; all the good guys are blonde, white, and blue-eyed. And maybe I could excuse that if it weren't for the fact that THE AFOREMENTIONED GOOD GUYS ARE FROM AN EMPIRE THAT SPANS CENTRAL ASIA AND MESOAMERICA. I was even willing to suspend my disbelief a little and pretend that these jokers were improbably bleach-haired East Asian dudes, but Ms. Norton kept emphasizing their BLUE EYES and the fact that none of their stuff looked 'ORIENTAL'.
Your sci-fi epic should not bring to mind the "omg you can't just ask people why they're white" scene from Mean Girls, but that's all I could think of the entire time. When you couple that with the fact that the "heroic" faction uses torture and the fact that it brainwashes and mind-controls our protagonist without his consent... well, even the eldritch beasties that showed up in the third act couldn't take the bad taste out of my mouth.
If you really, really, REALLY like time travel stories and/or Atlantis, and you can find this for a couple bucks at a thrift store, then sure, maybe give it a look. The writing is relatively solid, and I was interested enough in the story to see what happened next, racism and questionable "heroes" notwithstanding. I'm also not against checking out some of Ms. Norton's other work, in hopes that it's maybe less of a product of its time.
But otherwise... well, the story that's there isn't really worth your time.