Plot is firmly into "making no fucking sense" territory, glad that's over with.
We know that the 10 original ATACs were found in some ruins. We also know that apart from select few scholars, medieval folk were mostly dumbasses. I think some yokel who was bored with living in a village in the middle of nowhere just happened to stumble over those, told his family about the discovery and they all set out to harvest the metal curiously shaped into deathmachines to smelt horseshoes out of it.
Two machines were already completely lost, the family of the guy who found them had already made quite a profit, when someone got jealous of their newfound wealth. He looked for opportunities to get back at them, try to find their source of income. So he spied on the original discoverer, as he and his two older brothers set out to completely take apart and make nails of the third robot. When he found the hidden way into the ruins, he went down there after the three guys had finished and found the robots.
He, being an asshole and less of a practical thinker, didn't see the opportunities of having a lot of precious metal fall into your lap, but instead one to harm his fellow man and rushed back into town, towards the local church-like institution and told the priest that those suspiciously now wealthy family was off daily into a cave to worship idols.
Subsequently, those "idols" were discovered. The whole family was put under torture, confessing...absolutely nothing, of course. It caused quite a ruckus, because the local priesty believed that they were so tight-lipped because they had become THAT avid followers of the strange heathen religion that nothing could break their devotion, not even torture.
They brought in the higher executives to deal with the problem, because a religion of that power growing silently to compete with the established one? No, Sir. Soon, a lot of higher-ups from the church came to the cave, saw the "idols", heard the family's cries for mercy, and decided well, let's cut the problem off at the source and destroy the things.
It was then that they discovered that while smashing the robots up with hammers in order to more easily transport the metal was covered under the past civilisation's security measures, setting them ablaze in a bonfire was not.
A minor nuklear explosion of the first burned robot later, with a lot of important people eradicated, somebody with half a brain figured that this was not the sign of some higher power, but instead a serious opportunity to really smite disbelievers.
He immediately ordered to have them analysed, taken apart very carefully and understood. Fortunately, six completely intact robots were left. And we're talking about the equivalent of the Catholic church of our world here; they had all the power, they had all the knowledge, it was them who were keeping it from the general populace, after all. Everyone who had talent in anything could only hope to use it for practical purposes after studying in a monastery or a church-led university...meaning that they had the finest minds of their part of the whole world to set to the task.
And copying stuff you don't even remotely understand is definitely easier than coming up with it yourself, that much is sure. There were a lot of setbacks - cables just don't work when you try to make them out of hemp strings - but it didn't take that long for them to catch on that this needed to be metal, this needed to be NOT metal (an isolator), etc. ...
The power supply, of course, was a different problem. Fortunately, it was soon discovered that the reason those things worked at all was in those handy spheres who glowed in the dark once you removed the outer hull (except in the cases when that led to a spectacular explosion and a setback of seven months) and gave the people working on it life expectancies of mice. And THOSE had been left behind by the ancient civilisation pretty much everywhere. People were digging them out like they do here with spearheads. They were sold as lucky charms. Crafty assassins hid them in fireplaces, causing whole buildings to blow up when the heat got high enough (for the machines back then had excellent cooling systems, the power generators didn't HAVE to be heat-proof over 300 degrees).
With that problem solved, after five years of pouring every ounce of effort and creativity they could muster, a working prototype of the first ATAC was submitted. It could walk three paces before the power cell ran out, and lift its arm until the misproduced joint broke. But they knew it would eventually work...for it was a gift of their deity that they had discovered the robots at all.
And it did, some more years later. By then, plans had been leaked. Scientists from all over the world had come to work on the machines, after all; they didn't stay with the project indefinitely, they went home...and some were patriots more than faithful believers. Soon, the whole world had the robots. A new age begun.
And this is why the robot on the game's title screen has a cross on the forehead.