Conversation so far:
What was Basel?
Holbein bought a house in Basel, in St Johanns-Vorstadt.
What did he use the house for?
He presumably returned home to preserve his citizenship, since he had been granted only a two-year leave of absence.

Question: What was significant about the house?
Self-contained rewrite: For Hans Holbein the Younger, what was significant about the house in Basel?