In a spot that's always barred.

this may be no more than referrence to his barred cell...

Coming late to this spot, I believe this refers to the early days of English billiards when you could initiate high breaks by directly pocketing the red off its own spot - and at some time in the development of the game (quite likely around the time that The Mikado was written) a new rule was instituted which barred this action. I once played on an ancient table complete with mahogany score board marked something like Orme & Co Spot-barred Billiards, but of course Joe's right about the allusion to barred windows as the pun.

~ Billy Hardball