There was a wide variety of doors and windows to be seen, some looking more authentic than others, some being very evidently modern. And of those modern ones some made a better job at concealing this than others.
I think that maybe the two doors at the castle gate (Alte Kanzlei) were truly authentic, or at least very old. Wood and iron can survive centuries when kept dry and these both were good hidden from immediate reach of the elements. The one on the left was on the back side of the building, the red one on the right on the inside of the gate arc.
What I found interesting however is the aforementioned mixture of authentic looking and modern. I am not very well informed on this problematic, but to the best of my knowldedge in CZ if someone owns an authentic historic building that is listed as such and is protected by law, then all repairs must be performed with technology authentic to the time the house was built – at least on the outside, or on the specific part that is being protected. Once I have talked with an old lady whose house had authentic wooden shingles on the roof and she complained about how difficult – not to mention expensive – it is to find somene to do repairs that are confirming with law.
It seems that in Idstein not all houses have such strict protection , although some perhaps do. There was definitively a visible hint of plastic here and there.