I'm currently playing a game where I cannot build Cavalry or found Sid's Sushi, and I do not know why. I could build Knights, and I thought at first it was because I had skipped Hunting clear up until the Industrial age, and that it was some kind of hidden prerequisite for Cavalry, but I eventually researched it and Archery and I still couldn't build any mounted units better than Knights. I can build Riflemen, Cannon, friggin DESTROYERS, and I know I have all the prerequisite techs, but I can't build Cavalry (or Cuirassiers, for that matter). Then, I finally got Medicine and went to found Sid's Sushi, and it wouldn't give me the option. Checked, it has not been founded yet, I have Corporation and Medicine and a Great Merchant, no Sushi. It's like the game deliberately disabled two of my favorite tools of the latter part of the game.