When Jeremy Irons was asked why he did Dungeons & Dragons (2000), he replied "Are you kidding? I'd just bought a castle, I had to pay for it somehow"
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The same way most people do, with a mortgage. The difference is what a bank is willing to lend you if they see you have a significantly higher than average income.
It's also possible he wasn't just talking about the purchase payment. Large, old, valuable buildings also often require very large upkeep bills.