RE: The picturesque architecture of Brühl, Germany.

No idea what the symbols are! I did some random sleuthing. Put it into google lens, got the text, and put it in chat gpt with your context. This is what chat gpt said, and it seems to be confirmed here.

In the context of being on a door in Germany, "20*C+M+B+16" likely represents a traditional custom associated with the Epiphany or Three Kings' Day (Dreikönigstag). The markings are typically made with chalk and include the year and the initials of the three Wise Men, Caspar, Melchior, and Balthasar (C, M, B).

The formula is often written in the format "20+C+M+B+YY," where "C," "M," and "B" stand for the initials of the Wise Men, and "YY" represents the current year. The "20" at the beginning and the "+16" (or the current year) at the end are constants.

For example, if the current year is 2023, the inscription might look like "20+C+M+B+23."

This tradition is a way of blessing a home and seeking protection for the people inside it. The formula is a combination of religious symbolism and a blessing for the household.