I hope that doesn't end up being the case of how technology ended up on Eternia.
I always like the idea that eons ago, Eternia had great science and technology that was abandoned after the great wars that nearly destroyed the planet.
I always thought that the Age of Science predated Castle Grayskull. As Eternia is the center of the universe (or dimension) the planet itself is the power source and at some point, all of that power was channeled into a gateway which is where Castle Grayskull was built. Whether or not the Castle was the original gateway or not, who knows. But we know to control the power you need to control Grayskull.
In Adam's time - Eternia is on the cusp of a scientific renaissance, led by Duncan and other Eternian scientists. The secrets of the Age of Science are still being rediscovered. Reverse engineering of Horde tech could certainly assist in that quest, but I wouldn't want tech on Eternia to come solely from the Horde.
My Chronology:
Early Times
Industrial Age
Age of Science I
Great Wars I (ends Age of Science)
Dark Ages I
Preternia (King Grayskull, HeRo, Rise and Fall of Snakemen, First Attack of Horde)
Great Wars II (ends Preternia, death of KG, Horde Defeated)
Dark Ages II (Oolar, Wun-Dar, Tales of Zodak, Council of Elders)
Great UnRest (Miro's time)
Prophecy Age (Randor/Keldor conflict, Birth of Skeletor, Adam becomes He-Man, Adora rescued from Despondos and becomes She-Ra. Horde returns, snake-men return)
Age of Science II (New Adventures, technology rediscovered. Eternia no longer a hunter/gatherer civilization. New golden age of peace on Eternia)