I'm a total noob to this forum, but I've played Warband a fair bit, and have been playing this mod quite a bit lately. I enjoy it, for the most part, and I really appreciate the work that went into it and AD 1257. Having said that, however, I felt compelled by the comments praising the "historical accuracy" of this mod to remark that, in my playthrough, I've noticed some very serious inaccuracies.
First, the two Princes of the HRE, Johann and Heinrich. Johann would no doubt be Richard of Cornwall's first son, John; unfortunately, he died in infancy in 1232. Heinrich must be Richard's second son, Henry of Alamain; however, apart from being present for his father's coronation at Aachen in May of 1257, and subsequent knighting there, he spent basically no time in the HRE. Indeed, the German Wikipedia article about him states that he left Germany and had returned to England at the end of September of 1257, which makes sense given his later involvement in the Second Barons' War; we know that he was present at Oxford Parliament in 1258 when the Provisions of Oxford were promulgated, so he couldn't have play a role in the affairs of the Empire. It's also unclear why Henry is nicknamed "the Kind;” as far as I'm able to determine, this was never a nickname he had in actual history.
Second, I’ve noticed that none of the fiefs in the HRE match up with their historical owners. I don’t know if this is the case with others, since I’ve only played the HRE so far.
Third, a number of fiefs are not in their proper locations. Most of them are relatively minor, but several are badly out of place, based on their names. To wit, Neuß is on the wrong side of the Rhine, Friesach was not in the Rhineland as it is in this mod (it was in Austria), and the location occupied by Straßburg is actually Pordelone (which belonged to the Patriarchate of Aquileia; there was a Straßburg in Austria, which, ironically, was not that far from Friesach, but that was on the other side of the Alps).
Fourth, the way the factions are set up is really inconsistent (and inaccurate). There were more Crusader States than belong to the Crusader States faction (e.g., the Duchy of Athens), and not all of the fiefs in the Crusader States faction were part of the same polity. There are several that weren’t even part of actual crusader states, but belonged to Byzantine successor-states (the Despotate of Epirus, the Empire of Achaea, and the Empire of Trebizond)! Also, the Armenian Kingdom of Cilicia was a thing; it was a protectorate of the Ilkhanate, but still mostly independent, and the Principality of Antioch was a vassal. The Gaelic Kingdoms were a collection of petty kingdoms. Bohemia and the State of the Teutonic Order were fiefs of the Holy Roman Empire, but they’re independent factions in this mod. The Kingdom of Mann and the Isles, which controlled the Isle of Man and the Scottish Northern Isles was de facto independent of the King of Norway. Silesia was de facto subordinate to the Empire at this point, not Poland, and was a collection of independent duchies (three in 1257, but it underwent refragmentation and reconsolidation).
Fifth, there are several fiefs that, in this mod, belong to the wrong faction, even without accounting for the inconsistencies noted above. Most of these are on the eastern frontier of the HRE.
I'm compiling a document correcting these problems, which I'll provide on request when I'm done.