The current idea is that the city itself is built into the ruins, and only some of the ruins (most of the city area, minus the fringes / underground) are safe. The ruins used to be a sprawling port city, the surrounding environment outside the city is jungle but the wilderness has been slowly growing over the ruins so a lot of the outer ruins are pretty overgrown. The civilization that originally built the ruins was extremely powerful, and some of their ancient spells are still in effect, doing such things as holding towers or large chunks of land floating above the city. Some of these have been reached and inhabited, usually with rope ladders and bridges, others haven't been able to be reached / entered yet and are still total mysteries.