The most close to ancient nords structures?

The Pale, Winterhold, Windhelm, and The Rift are all referred to as the old holds. Assuming you're speaking of the ancient nordic dwellings and their architecture, then Saarthal is going to be your oldest example. Labyrinthian sits on land that was once the Bromjunaar Sanctuary. If you go there with the eight dragon priest masks, then you can travel into the past and see what an interior dwelling would have looked like near the era of dragon worship. It's hard to say, off the top of my head, which of the four holds could possess architecture nearest to that of the ancient nordic dwellings. I can say for sure that Windhelm hasn't been remodeled in a long time (disrepair). The nordic barrows seem to be composed of stones, so that would rule out some of the wooden architecture in Riften and Dawnstar, and might be another point in Windhelm's favor.

So I guess my bet is Windhelm.

There is a theory (although I may be the only one that believes it) that the nords were born from the Throat of the World. If you want really old architecture (ie. before the return) you could have something on the slopes of, or even within, Snow-Tower.

Windhelm is considered the oldest city and skyrim and used to be the original capital. Anything older than that you would see around in old nord ruins

Ok because the actual nordic structures look quite different rom windhelm and all other nord cities apart from some carvings and dragon motiphs , Have you seen the citadel concepts? this is a really cool structure but I guess is not ingame right?