Moisture in the exterior wall of a 300-year-old house

Unfortunately not.

In the walls, the moisture has even risen to a height of almost 2 meters.

I think such rubble stone walls should not be plastered at all and left as exposed stone walls. The stones themselves do not absorb water, only the joints and, of course, the plaster do...

So remove the old plaster, dry with large infrared heating panels. And then always keep the rooms slightly heated.

But with such thick walls, you won't find a proper method to keep them dry, like a horizontal barrier or something.

But maybe I am wrong??

No, you are right. Such old masonry is never truly dry by nature.
A few years ago, an architect explained this to us. I am a churchwarden, and we were annoyed that the plaster inside our 800-year-old church kept falling off. He said roughly that the saying "a house must breathe" is usually nonsense, but for buildings this old, it actually holds true. Built of brick, very porous, made of fieldstone boulders, with shell limestone mortar, completely without cement, the house already draws moisture from the surrounding air, and if, like we had done, you apply cement-lime plaster inside as in any new building and paint it with Caparol, you create a barrier inside, and the old masonry rejects this, the moisture makes the plaster detach. We then, under his guidance, had the old cement plaster removed from the walls, applied a type of trass renovation plaster, had the walls painted with a lime paint from Keim, and arranged the grounds so that there is a slope away from the house, and look, the plaster holds, the walls feel clammy but it is not unpleasant to be in the building. Karsten

Well, living in something like that just means you have to accept an unpleasant climate, or spend a lot of money.

The depiction of the "unpleasant climate" shows ignorance.

There are many methods to get such a masonry and consequently the floor dry. Horizontal and vertical sealing methods, injections, drains... Something like that has to be evaluated by a professional. You can easily spend 50,000-100,000 on something like that.

Please explain why the house has been standing for 300 years and other buildings have existed since the 14th century. Walls used to be somewhat damp – that was part of it and was generally not a problem because of the leaky windows and the associated air exchange.

worm goblin, in some posts ignorance is paired with the tone of conviction. must be ignored.....K

My parents-in-law live in an old parsonage from the late 19th century. Built from various materials ranging from bricks to large boulders in the base area. Fifteen years ago, the outer walls were also mostly damp after it had been vacant for a long time (but not flooded). The walls are also about 60 cm thick.

Renovation was done using cement-free lime plaster, new windows were installed, and the surrounding ground was slightly lowered so that there was still a slope away from the house. However, no additional drainage was installed. On the weather-exposed side, the wall was still damp in the base area even after the renovation. Then a baseboard heater was installed and additionally a ventilated facade was added. Now everything is fine, the house is very pleasant in terms of indoor climate, but the heating costs are certainly not low – but the house is correspondingly huge with high ceilings, in line with its former use.

Yes, those are really mansions... you don't ask about heating costs there. You have your own forest for making wood. - Karsten