Did I catch the rule correctly that once you play something other than the longship you can't use it in the later acts of the game?
I'm not sure how else to interpret the rules:
rules about Ending the Act wrote:
Longships return to the player’s pile when scored. They are
available to be used again in future acts. Castles, Thanes, and
Beowulf, on the other hand, are removed from the game
when scored, so return them to the box.