Could Lando have repulsed an Imperial attack

No. The Empire had more force to continue a relentless assault, and it's not like a city can move.

The Empire would merely need to cut off their imports, as well as destroy the one pole they stood atop of.

All easy enough for the Empire.

No

No. I don't even think he could have repulsed the Executor. The Imperial Fleet would have butchered Cloud City.

However, I don't think he was defenceless, and he might have been able to put up some kind of resistance. I say this because Vader negotiates with Lando. Now, granted, when he starts "altering the deal" it's clear that Vader thinks nothing of Lando, but if the fight would be totally one-sided, why barter at all? He obviously wanted to make Lando think that this was a deal - maybe with a little threat - but that wouldn't work if Lando didn't believe he was holding any cards.

Theory No.1: Cloud City has a planetary shield like we've seen on Endor and the Death Star. While this wouldn't repel an Imperial assault, it would cause trouble, and Vader didn't want another Hoth fiasco where Han and Co manage to slip through a blockade.

Theory No.2: Bespin is - at least in novels and other materials - known as a gas mine, used in laser tech. Perhaps the Empire relies on this resource and wants to play nice with Lando, who could theoretically ignite the whole planet or otherwise taint their main "ammo" source.

But the real reason...

...is the fact that Cloud City has the facilities to carbon freeze Luke. Vader wants to test the facilities on Han, then freeze Luke. So he pals up with Boba Fett (who can get the bounty on Han) and negotiates with Lando because Vader really needs the facilities, and for the encounter to be subtle - not a firefight. Of course, it's Star Wars, so it turns into a firefight, anyway...

No. Not just because his floating city could be shot down by anything with sufficient firepower, but because having a fleet of battle-ready spaceships would have been prohibitively expensive. Lando was running a for-profit opreration, and that probably meant enough defensive capability to repel boarders, but not enough to shoot dreadnought out of orbit.

And that's why Lando caved the minute Vader's ship came to town.

No chance. He's far too sexy.

No, no chance.