Thanks. But I haven't conquered anyone. I started off with one - tribal - holding (a village/small town), and my government is definitely tribal, but I still have "feudal" and "city" law options...
They still give you the option just in case something weird happens. For example, I conquered Iceland as an Irish petty king (mostly to keep from having to deal with an additional source of Norse raiders), then upgraded to feudalism, meaning both counties got free castles. But then I was forced by a faction to grant independence to the Earl of Vestisland and Countess of Austisland, and in the interim between then and my reconquest, Austisland fell to a Swedish county conquest.
Because Sweden is tribal, they get penalties for directly holding the castle, so they had to build a new tribal holding in the empty slot and make it the capital. Then the Swedish jarl in charge appointed a Þegn (baron) to rule the castle. Boom, suddenly the jarl is getting feudal taxes.
TL;DR: The laws for city and feudal taxes exist for tribes
just in case you conquer any.
