Perhaps the most essential Crusader Kings 2 mod available for those after a challenge CK2+ changes host of things about the base game with the aim of 'deepening the CK2 experience'. Have you ever tried Rome I - Roma Surrectum II? (More.).
the problem of being a series of such a scale and that one game Rome 1 and Rome II have multiple years of development in between is a huuuge problem! (More.)
