Battle of the Goldroad

The Third Battle of Tumbleton is an engagement in Daenerys Targaryen's invasion of Westeros, in which Daenerys Targaryen's Dothraki horde engaged the main House Lannister field army as it was returning east to King's Landing after successfully sacking Highgarden. It was the first time that Daenerys committed one of her powerful dragons to battle, personally riding her mount Drogon into combat.

Prelude
Tumbleton is a natural choke point for any army advancing up the Roseroad from Highgarden in the southwest to King's Landing in the northeast - located in the northeastern corner of The Reach, amidst hills and cliffs around the source of the Mander River. While the Mander flows southwest, a short distance farther east is the Blackwater Rush, which itself flows east to the sea, with King's Landing at its mouth. The Roseroad passes Tumbleton to the south, almost reaching the sea, until it reaches the southern bank of the Blackwater's mouth - at which point an army needs to cross the river to reach King's Landing on the northern side.

Due to its strategic location, the last major hurdle for an army heading east on the Roseroad to King's Landing, Tumbleton has thus seen a number of major battles throughout history, notably the first and second battles of Tumbleton, which were fought during the Dance of the Dragons - and both of which involved dragons burning out entire armies.

After the Fall of Highgarden, the bulk of the remaining Lannister army advanced east led by Jaime Lannister, along with Randyll Tarly commanding his own Houses's formidable forces.

In the books
In the A Song of Ice and Fire novels by George R.R. Martin, an equivalent battle has not occurred. The TV show has surpassed the timeline of the books, so it is unclear if an equivalent battle will occur or not.

The battle hasn't been given an on-screen name as of Season 7's "The Spoils of War". "Third Battle of Tumbleton" is a more formal (albeit placeholder) title for a wiki-article than "Daenerys and the Dothraki attack the Lannister army in the northern Reach", etc.

There are several indications that the battle actually takes place near Tumbleton:


 * 1 - It takes place near a river, but isn't within sight of King's Landing. Also the Roseroad enters the Kingswood as it approaches the southern bank of the Blackwater, but no woods are visible during the episode's battle.
 * 2 - Randyll Tarly says that they need to get their grain shipments across the Blackwater to King's Landing as soon as possible, so they are relatively near King's Landing, not Highgarden, but still on the southern side of the Blackwater.
 * 3 - Tumbleton is located in the hill country where the headwaters of the Mander River spring from, and historically, it has been the last major geographic obstacle for armies heading east from Highgarden to King's Landing, thus a natural choke point where enemy armies tried to cut them off. Hills and cliffs are visible in the episode's battle, and on maps, the hills which the Mander springs from are really the only elevated terrain anywhere from Highgarden to King's Landing.

If the events of Season 7 have some basis in the future novels, it is possible that George R.R. Martin meant to foreshadow this battle when he wrote his extensive history of the Dance of the Dragons, the great civil war between two rival branches of House Targaryen in which both sides had dragons. Not one but two major battles were fought with dragons at Tumbleton, explicitly to attack armies which were heading to King's Landing. During the Dance, the climactic Second Battle of Tumbleton was a massive battle royale in which all of the surviving dragons in the war fought and died, in a chaotic night-time assault illuminated by their fires, as thousands of men died in the mud and smoke around them.