Instructions
Travel to the back gate of Aisha's castle while carrying a Carbon Parchment. Use the parchment to make a copy of the cipher located on the gate. Take the copy of the cipher back to Nythera in Guwio Village.
Find Nythera and show her the copy of the gate's cipher. She will instruct you to seek out and interrogate one of Aisha's minions located in the Frozen Northlands. Leave the village and head to the Frozen Northlands.
Defeat either a Snow Golem or Frostwrym Rider to receive the first number in the combination. Return to Nythera with the first number and she will instruct you to find three more minions to retrieve the final three numbers of the code. Go back to the Frozen Northlands and defeat three more golems or riders in order to get the final three numbers.
Visit Nythera with the combination. She will then send you back out to the Frozen Northlands one final time to retrieve the Rosetta Rune, the component that is required to decipher what rune each number represents on the gate's cipher. Kill any Snow Golems or Frostwrym Riders you encounter in the Frozen Northlands until one of them drops the Rosetta Rune. Use the rune to unlock the gate and complete the Sneak into the Fortress quest.