Old LA and New LA (Lion's Arch)
Shortest version: Season one of the Living World.
Short version: An insane sylvari genius conned antagonistic factions from all over Tyria to attack the city while she got at the hub of ley lines under the harbor and redirected them to wake up the Elder Dragon Mordremoth. She died in the process, and her minions were eventually driven off, but the city was razed to the ground in the process and eventually rebuilt in the form you see today.
Mind you, it was the climax of a plot that ran for 15 months, so there’s some oversimplification happening there.
After all Aaron said, they eventually decided rotted retired boats were cute for a pirate theme, but didn’t make for a very defensible city. So they decided on more concrete structures.
And since the Consortium had control over the design, they made it touristy and theme park-like.