I hardly believe your city was created through secession.
it is city where i live now, not actually my city. and yes, when large-empire sovereign imposed too heavy taxes on them, they declared his rule null and void which equals secession, beforehand they made heavily defended city fort, and established good working diplomatic ties with others in surrounding, which created noble republic eventually, beginning at about 1200's, finishing with Napoleon occupation.
as far as i know there are many examples how cities are fighting with royals over taxes. many royals asked cities to accept some level of taxing to avoid military occupation and overthrowing of local command, and if we want to have realms as political facts, city taxing would be their major power.
and that has nothing to do with modern time nationalism, rulers were spreading their rule through either conquering or agreeing taxes with cities, which particularly applies to city in Mediterranean surrounding, which were mostly formed in roman times, some of them even in greek times, and medieval rulers appeared at very beginning of medieval times, where sign of their presence were taxes and army service obligations imposed, and who opposed that had to fight and either lose command or obey.
actually, "nationalism" existed over whole middle age, where Holy Roman Empire is - to my limited knowledge - the most extreme example of weak rulers vs. strong dukes, and even there rulers spent centuries in constant struggles to gain more power.
historically, the never manage it and Westphalia peace was ultimate admission of surrender, but if we are recreating history in BM, in simplified manner, we should be allowed to recreate that conflict.
maybe i can say it in simpler manner: in rl, dukes were always stronger than rulers holy roman empire, but
struggle existed all the time, and there would be no struggle if rulers knew they will never win over dukes.
if devs announce duke's historical victory in advance, than there is nt struggle, history is resolved in advance. and that very much explains lack of conflict mentioned in some other posts. dukes
were more powerful than rulers, but they had to fight for that power
all the time. and they won in holy roman empire, they did not win in french realms, nor in east european realms, while in england they got even more, later, so all kinds of outcomes were present, and all-time internal fights were present.
is it not good background for bm, where proponents of ducal power should fight proponents of royal power
all the time, in-game.
that is my thoughts only...