This sounds like a question for our posters who have chosen careers in the military... Tywin, josh, or even Aegon, though he's not been here in four years or more.
My opinion is that Braavos is a logistical nightmare. It is far removed from most of the other Free Cities so that supply for an overland assault would be a problem. The land approaches are through hundreds of miles of uninhabited rocky wilderness, but the final distance is through bog and marsh. And so, the Braavosi have not bothered to defend the landward approaches... their entire defense is naval based. They have a fleet of harbor galleys if anyone actually gets past the Titan.
But Braavos' location means it is also impassable from the inside. I don't know if the defense of the city would be stiffer knowing that there is no where to retreat.
In the above pic, you cannot see it, but under the Titan's girdle are a hundred murder holes from which burning pitch can be dropped on ships. And there are dozens of ballistas and scorpions in the legs. I think I remember, from Arya's arrival, that there are forts guarding each of the Titan's legs. So any attack on Braavos must first consist of an amphibious assault to capture the forts, then capture the Titan, and then force a passage against the fleet all while keeping weapon, ship, medical, and food supplies going to a remote location.
The best option to carry the city would seem to be by aerial attack. If Dany does not mind civilian casualties, then dragons could cripple the city's will to fight and probably destroy munitions, food supplies, and even ships in the dark.
I know the Free Cities constantly fight with each other, but they really don't want outsiders conquering any of them. If the Westerosi moved against Braavos, I'd expect to suddenly see an alliance between Myr, Lys, Tyrosh, Volantis, Norvos, Qhohor, and the other two cities whose names escape me. Even if they did not prevent the fall of Braavos, their combined economic boycott and state sanctioned piracy on Westeros could be catastrophic.
Yog, by the way we had a discussion, years ago, regarding how the Eyrie could be attacked.
Found it...
The Eyrie.
Research Monkey happy now.