I’m not asking for help. I was just about to ask for help, but I figured out my error. FYI, I’m on the Windows version of AOE IV.
The Holy Roman Empire “Fleet of Foot” Mastery is to “Research Marching Drills and produce a force of 30 infantry in the Castle age (III)”.
If you’re having trouble with this Mastery, you may be making the same mistake I did. I tried three times to get this mastery without success before I figured out what I was doing wrong.
I took a closer look at the achievement. When I hover over the information icon at the end of the Mastery title, it says to research Marching Drills at the Blacksmith. The three times I tried for this achievement, I was researching my blacksmith skills in that Meinwerk Palace.
On the fourth try, I built a blacksmith and researched Marching Drills there. This time, I got the Mastery. I researched Marching Drills in the Feudal Age (II), but that didn’t seem to matter.