The letter from the Gatekeeper at the end may hold some secrets, or maybe a new puzzle.

Possible spoilers?

The letter is very peculiarly worded. One pattern is that there are several places with the pattern "(word starting with a letter foo) (word) (another word starting with letter foo)": 
Leaves a lasting 
Within its walls 
Each evildoer exercising
conquer your challenges 
corner ever can 
and I am

In addition, there is the conspicuous "play fair" in the middle, suggesting a Playfair cipher of something. The diagonal direction of The Gatekeeper's face may be significant too.

The final encounter with the Gatekeeper in the end of the Theseus guide essentially says that the Gatekeeper letter reveals where the Gatekeeper is going next, and the Gatekeeper gave the key to them. Presumably we interpret something in this as a Playfair cipher.

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