Based on the information provided, the answer to the cryptogram “opening in een vestingmuur” (4 letters) is bres (4 letters).
- Meaning: Bres translates to “breach” or “gap” in Dutch, specifically referring to an opening in a fortification wall.
- Number of letters: Both “bres” and “opening” have 4 letters.
This aligns perfectly with the clues given in the cryptogram.