So I'm writing this story, the premise of which is: 16th century technology people find giant war mechs, 9-15m tall, with appropriately sized weapons (20-100mm cannons and rockets, as well as non-chemical weapons that they can't understand like DEWs and railguns) attached to them, buried underground. I probably won't actually talk about this in the story because I'm not confident I can give a plausible explanation, but let's say I gave them a quarter century to reverse engineer the mechs and the weapons. They understand propellant and have examined and co-opted the idea of cartridge ammunition, and they've also adapted the breech block technology of modern artillery. They can reproduce both of these technologies (with one caveat: cartridges don't have built in primers yet), however, they lack the industrial capability to mass produce any of these things in significant numbers, and it's in these early days when ammunition and cannons are scarce that mechs find their place on the battlefield as formation-breaking cavalry.
I want the mechs to have large crews. How can I justify this? The mechs must have some kind of mechanical (i.e. non-digital) way of piloting them that medieval people could plausibly work out given enough time. How would you even describe the controls for a bipedal walking tank? If I can solve these problems of describing the mech's interior and what the crew is doing inside, I can actually write the story. For now I'm just refining my ideas for plot points and characters, and just drafting the outline over and over and bulking it up into a summary.