Yeah thats the critical point - the civ you design could be anything! From Faladamir 'The Great' ibn Sviatoslav, the early Rus' ruler who made his name by converting to Islam and facing a punitive war against Byzantium in which he rallied the people of the Rus' Sultanate against the infidels, cementing Islam as the dominant faith of the region. Or, on the other end of the spectrum, the Islamic Republic of Russia in 1991, a country gripped by the collapse of its holdings in the East and facing a Orthodox Christian insurgency in the Southern Caucasus.
Could be a Teutonic order on Steroids which, bolstered by recruits who signed up to fight against the Muslim infidels, captures Novgorod and Pskov before marching on Moscow. Could be a crusader state carved out of the Crimea; the Kingdom of the Bosphorous, forged at the height of the crusades to fight against sufi Cossacks, the mighty Kievan Sultanate and raids from the infamous black sea corsairs.
I'd hardly say its restrictive.