Tirion Isilmírë, the "Tower-City of Deepsilver", was the capital and only true city within greater Xilvalur. More of a series of forts than a single city, it was the oldest wood Elf bastion in the world. Despite stretching vast distances across high mountain peaks and amidst dense forestlands, it was was hidden from the Wyrm's Spur by illusory magic, resulting in very few curious inquisitions by those traversing the bay. It was insulated from the Sea of Trees by the Wood of Ara'mírë, and watched over the forest through magical alarms and Scrying sensors.
Due to the sheer breadth of Tirion IIsilmë, Xilvalur was considered to be a nearly-impregnable nation, as it contained the Epochal Artifact Lûthloth Mîr. Over eons, elven druids worked alongside the artifact to completely alter the minds of interlopers, cast abujrations that shielded its walls, and create Portals between one fort and another. Generations of rangers watched its borders, ensuring that no horror from the deeper woods would breach its towering wooden halls.
The city was composed out of living trees shaped into elegant spires, but when needed, some of these trees could uproot themselves to defend the city. This included the central Tree of Teralis, the largest tree in the known world after the Elfintree; if, somehow, creatures made it through its litany of magical defenses, they would be crushed to oblivion under the roots of Treants and Animated Trees.