Old Oct 13, 20, 8:29 pm
Join Date: Sep 2011
Location: MIA
Programs: AA Dirt BA Gold SPG GLD HH GLD
Posts: 901
I have never heard fortress hub in the context of upgrades, just normally in the context of one airline dominating the majority of flights.

Off the top of my head I can think of:
AA in CLT, PHL, and DFW
UA in EWR, DEN, and SFO
DL in ATL, DTW, MSP, and SLC
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