Canon from Haibane Renmei
The town of Glie and its suburbs are surrounded by a wall, this is indeed the single defining feature of the town.
The wall is hazardous to haibane; touching it can lead to the haibane in question becoming fatally ill, and new feathers are warned against even approaching it. It feels cold to the touch, but since few touch the wall and even fewer survive touching it, this is not widely known. The initial symptoms of wall sickness are a false warmth and anti-fever (the victim feels intensely cold to the touch to others); it is also a very effective painkiller and will numb the sensation of any other injuries the victim may have. Within a few hours, the anti-fever turns to a raging fever that may be treated only by an herbal formula grown in the Haibane Renmei Temple.
It was apparently forged in one constuct and then penetrated for interior and through access in at least two places (the Temple and Great Gate.) The wall contains a hollow space with a moat which may be navigated by boat and accessed through a passage from the Haibane Renmei Temple. A grey armored robe protects occupants from the wall sickness, and dialogue in 'Haibane Renmei' implies the existence of possible enemies. Lining either side of the moat are catwalks with tags engraved in the language of the [Toga]. This language also occurs on the suit, boat, and books in the library that are apparently made of stone.
The wall is perfectly circular (unlike in the 'Haibane Lifestyle Diary' map), ten kilometres in diameter, and just over 100 metres tall. Much about it has been forgotten, including how it was made and why. It survived unattended for at least a thousand years, but is now looked after by Tatakai http://haibaniki.rubychan.de/wiki/FHD_Remix; she knows a lot more about it.