I just had a game where the Kingdom included Upgrade, Fortress and Royal Carriage.
To make matters even more amusing, Donate was available - so having bought a Silver, 2 Golds, Fortress and two Upgrades I could Donate - trash everything else (yes, I suppose I could have trashed the $ too, an error on my part!) and used the Double Upgrade-Fortress combo to pile the Upgrades and then pick up as many Royal Carriages as my opponent didn’t buy, queuing them all on the Tavern mat as they came up. The following turn, I had enough plays of Upgrade on Fortress to pile the Duchies, buy a Province for good measure and win 30-0 in 12 turns. Completely mad - I suspect the other player didn’t see what was going to happen until the turn when the piles started to empty.
Upgrade + Fortress: one worth knowing about!