Hosgunn Crossed-Daggers controversially ascended to Riften's throne in 4E 98 and instituted harsh taxes for his personal benefit. The citizens rebel and burn Hosgunn's castle to the ground with him inside, but the fire spreads to the rest of the city. Riften is rebuilt over the next five years, but remains little more than a "glorified fortress" of "wooden structures and rough stonework" compared to the sprawling city of old.
~UESP
I had my information wrong, it was just a pale shadow of it's former self.