To find Fortbyte number 27 of the decryption challenge, go to map location A4. When you open the map you will realize that it is divided into squares like a schedule. By combining a letter and a number it is possible to divide the map into smaller areas, easier to explore.
Square A4 is located at the western end of the map and is half covered by the sea. The Fortbyte is located in the middle of the square at the edge of the cliff.