Short answers, pulled from the story.
Crimean Tatar forces led by Devlet I Giray set the suburbs of Moscow on fire on the 24th of May 1571, and a sudden wind drove the flames through the entire city. The attack followed Muscovite raids on Crimean territory in 1556 and 1558, and the complete breakdown of relations between Muscovy and the Crimean Khanate by 1563.
Devlet I Giray, the khan of Crimea, led the attack with a combined Crimean and Ottoman force that included Tatar horsemen, 13,000 irregular Turks, and 7,000 janissaries. The army bypassed the Serpukhov defensive fortifications on the Oka River and crossed the Ugra River to approach Moscow.
Estimates vary widely. The Crimean ambassador in Warsaw stated that 60 thousand people died and an equal number were captured. The English diplomat Giles Fletcher put the death toll at 800 thousand, a figure historians regard as an exaggeration. Other sources record between 10 and 120 thousand killed.
The Oprichnina was disbanded following the fire, which exposed its failure to protect the capital. A number of influential oprichniks were executed in the aftermath, and the Oprichny Palace was never rebuilt.
In 1520, Moscow had at least 100 thousand inhabitants living in 41,500 houses. By 1580, the papal legate Antonio Possevino reported a population of no more than 30 thousand residents, reflecting the catastrophic losses from the fire and the raid.
The Belgorod Wall was a stone barrier encircling the area of Moscow known as Bely Gorod, conceived after the 1571 fire as a defense against Tatar cavalry. It was built in the early 1590s by the government of Boris Godunov, who was the brother-in-law of the tsar.