How did the Starks able to rule the north for 1000s years?

[daenerys snow]

The point is, whether the Wall was built 8000 years ago, or 4000 years ago, the Starks have ruled the North from that point onwards. They are the oldest House in Westeros, and the oldest unbroken line of rulers of any of the constituent members of the Seven Kingdoms. In fact, they are the ONLY one of the ORIGINAL Royal lines that still remain intact and in power over their Kingdom. All the others were replaced by Andals and Roynar.

The Storm King's line has died out. So have the Gardeners of the Reach, and the Casterlys and the the First Men line of Lann the clever. The Kings of Mountain and Vale were rooted out when the Andals arrived, as were the original Dornish line.

The River Kings are long gone, and the original rulers of the Iron Isles were also swept aside by the Andal invaders.

The only original rulers in Westeros that still remain, are the Starks.

aren't the hightowers descended from the first men and were kings who gave up their crown and settled for a lordship (over oldtown)?

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aren't the hightowers descended from the first men and were kings who gave up their crown and settled for a lordship (over oldtown)?

They were "petty-kings" who sided with the Andals when they invaded. I think the Daynes are pretty old, too.

The Tyrells, Tullys and Baratheons are all upstarts who made their bones by siding with the Targs at the opportune moment. Same with the Greyjoys, sort of; the line is pretty old but they gained actual primacy after the Targs let the ironborn pick their paramount house. I'm not sure exactly how old the Martells are, but they obviously combined with/submitted to the Rhoynar pretty early on. The Lannisters' ancestor displaced the Casterlys, and through marriage they're as much Andals as anything else. The Arryns were founded when an Andal defeated the last First Men king of the Vale.

In terms of constant, uninterrupted rule and blood purity, the Starks seem like they're easily the winners. They've held basically the same position since their founding, and have only been "just" wardens since Aegon the Conqueror (that's 300 years out of like 8,000). The "southron ambitions" are pretty new, so it's likely that the same First Men blood in the north just kept getting recycled whenever the Starks got married; there wouldn't have been much opportunity to get Andal/Rhoynar blood in the mix.