I may have discovered a possible bug. Playing Rome, turn 229 at the moment. I've been trying to research 'Law of Citizenship for Latins and Allies', but it's locked for me. It doesn't state any building requirement and vice versa I can't find a building in the browser that states it unlocks this research. Clicking it simply doesn't work, exactly like with a research that is actually locked. I think I've built every possible building from the military chains that could logically unlock it. Campus Martius, Legionary Barracks, Fortress, Auxilary barracks are all upgraded as far as they can and are now locked because of this and further research (which are of course also locked behind this one). I've also tried upgrading all civic things like the library, Palatium, courts, coinage. Nothing works. Sooo, right now a lot of units are locked (Auxilla units, Augstan and Imperial legionares), which is pretty .
Thought this was worth mentioning. Hope there is a fix!