Firstly, I'd like to point out that the only confirmed Man of Iron in 40k (UR-025) bears more than a passing resemblance to the Kastelan Robots. I'd liken it to if they were options in a character creation system and someone played with the proportion sliders. I don't think it's ever been confirmed that there's actually different kinds of Man of Iron, but I don't think it's too far-fetched.
Interestingly, Codex: Adeptus Mechanicus mentions that the Kastelan Robots have existed longer than the Imperium, implying that they themselves, as individual machines, have existed since the Age of Strife at the latest. The line reads as follows:
"The cohorts of the Legio Cybernetica have marched to war in the colours of their forge worlds for ten thousand years, but the Kastelan Robots themselves are older than the Imperium and some call no single forge world home."
This is direct confirmation from GeeDubs themselves that the Kastelan Robots have existed since before the Imperium, in all likelihood originating from the Dark Age of Technology. I'd say that, combined with design similarities to the only confirmed Man of Iron and the fact that the Kastelans seem to demonstrate at least some measure of autonomy (showing up on the spot to whatever battle they decide to participate in), makes a pretty decent case for them being at the very least based off of Men of Iron.
I'm very open to counter-argument. Indeed, I'd like to hear others' thoughts on the matter. I definitely think that the Kastelans are more than they seem.
Edit: Little bit of support to my argument, it's confirmed that individual Kastelans, not just the design, have existed longer than the Imperium. Quoting the same Codex, just a different section:
"The Kastelan Robot combines tremendous physical might with a rugged stamina that has seen it fight the Imperium’s foes for ten thousand years. Indeed, some of the relic robots have existed much longer still."
Edit AGAIN, because I really should have read more of the Codex before posting this: Kastelans are DEFINITELY akin to Men of Iron, albeit modified to be more easily controlled by the AdMech. Quoting AGAIN from the same Codex, a bit longer this time:
"The Legio Cybernetica can trace its origins to when early tech-savants experimented with artificial life during the Age of Technology. Ever curious, those early pioneers not only constructed vast hosts of automatons but also gave their creations the gift of independent thought. That decision was to cost them dearly, and the rest of Humanity with them."
Edited by Zephaniah Adriyen, 09 May 2019 - 05:14 AM.