Tirion

According the the Annals of Aman in volume 10 of The History of Middle-earth, the building of Tirion was begun in the Valian Year 1133, and completed in 1140. Translating Valian Years into conventional solar years, that means that Tirion was started some 3,469 years before the Darkening of Valinor, and took about 67 years to complete.

The name Tirion goes back to the earliest of Tolkien's tales, and originally seems to have meant simply 'tower' (being a reference to a single building in those early stories, rather than an entire city). In his Appendix to The Silmarillion, Christopher Tolkien associates the name with the element tir-, meaning 'watch', presumably in reference to its location watching the only pass in the Mountains of Defence.