Chants of Sennaar

After you solve the puzzles of the Frontier and learn 33 of the 36 glyphs (you will have two unconfirmed glyphs and one missing) you will need to ring a series of bells to remove a line of Red Guard Warriors to progress to the next level. Once you ring the bells in the correct order all Warriors will run to a central meeting location and you are free to roam the level undetected. Check out Fashion victim  to knock out a simple miscellanious trophy during this section and then head through the giant doors to an elevator room.

As soon as you enter the room, you will see and learn the last glyph (Music) and be able to confirm the last set of glyphs. Once you complete the Warriors language, proceed to the elevator to make your way to The Garden level.

Below is a full Walkthrough of all Frontier level puzzles, glyph cyphers and the singular miscellanious trophy mentioned above: