How are they propelling the mechs?
What part of the 16th?
If the mechs are something in the vein of spider tanks, operating with relatively simple oscillating chechanics in the legs, then the legs could have their stride lengths and/or speeds individually controlled with what amounts to a mechanical gear transmission.
It would have to be some kind of handwavey bullshit engine that doesn't need maintenance or refueling. Relying on 16th century people to invent gasoline is just too much. I don't want to throw fuel concerns completely out the window but these mechs need to be something you can turn on after thousands of years of no maintenance.
Sorry if I was misleading, it's not alt history. It's Not-Earth, focusing on people who are similar to 16th century men.
Forgive me, but what is it about having more than two legs that would enable a mechanical gear transmission? I did envision the mechs being bipedal, but I'm open to having any kind of shape and size so long as I can justify it.
Wouldn't it be more entertaining if they had reverse-engineered the railguns-?
No.
Then you could have ye olde sea battles with gauss weapons.
No! Ancient sea battles would be piss boring if there were gauss weapons. It'd just be a matter of who could zap the other guy first. Plus, how in the blue hell are they going to make ferrous ammo, let alone power the gun?
More problematic is the power supply, i guess electric power would need some more time to develop and steam engines would need huge amounts of coal.
The advantages of a huge armored machine (especially if you can get the high tech materials from the original mechs) is that it would be impervious to small arms fire.
To be clear, they're not inventing mechs, these mechs were discovered. The story is post-apocalyptic. These people are finding mechs their ancestors built before their civilization collapsed.
It doesn't even have to be anything more advanced than steel. Black powder firearms struggle to penetrate even body armor of the time.