Ok, that's interesting - it's the exact same fight I got to when I returned to the place after Radahn. Seems like Radahn is treated as a scripted event that overrides the default state of the castle. I first set foot in it after progressing the Alexander and Blaidd questlines a bit, so the castle was already in festival mode - largely empty when I got there other than the festival courtyard and two lions patrolling at the entrance. Wonder what specifically triggers it.
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