I didn’t know anything about this. Nederland does not have this kind of thing. After looking around a bit I learned a lot, and my conclusion is that these structures are graduation towers. Graduation is a concentration process, the word gradient comes to mind and I know that very well. The unusually high piling up of (thorn) bushes, I guess ‘tower’ is appropriate, even if it is not a typical tower building. Even the word house makes sense, looking at some of the sites.
I understand many are kept especially as tourist attractions and health resorts. Next time I’m in Poland I will visit one!