NWSiaCB said:
Exactly.
The Spanish Conquistadors fought with horses, steel armor, and had primitive arquebuses (although they mainly relied upon their swords as they were killing too fast to wait to reload) against the Aztec and Mayan empires equipped only with padded armor and clubs with obsidian edges that were completely incapable of piercing steel armor.
The Spanish won battles where the odds were stacked against them 500-to-1 essentially without casualties.
Hypothetically, if you for some reason HAD access to full plate mail, and were going up against some force with nothing that could penetrate that sort of armor, you'd be essentially invulnerable, barring getting so tired or so swarmed that you wouldn't be able to fight off people trying to rip the armor off of you.
To be fair the spanish weren't exactly all on their lonesome. A lot of other tribes joined up, the Aztecs weren't exactly the most popular people.