[sen]

The fifth level was a binary puzzle hidden in the colouration of the writing. Darker/lighter letters were 1's and 0's. The second part of the 5th level was that the other ring of writing had 3 "colours", which were the dots, dashes, and spaces in a morse code string.

The only reason the fifth level was "hidden" was that there wasn't obvious hints/clues pointing to it like the first 4, where each specifically hinted at how to solve the next.