Egyptian Mythology.
An afterlife location, roughly translated to the field of reeds. It is similar to the Elysium fields in classic Greco-Roman mythology. Osirus is said to live there.

Located east of where the sun rises, Aalu is an archipelago of islands covered in reeds. Those whose hearts are judged favourably during the weighing of the heart are allowed to begin their journey to Aalu--a paradise where they may remain for all eternity. 

But the weighing of the heart is only the first obstacle one must past in order to reach Aalu. The path is filled with perils and one must pass through many gates, each one guarded by dangerous demons.

Greek folklore.
Nerite is most commonly found among sailor folklore. He is the only son of Nereus and Doris, and brother to 50 sister (the Nereids). Nerite is an extremely attractive and charming youth, which unfortunately lead to him being turned into a shellfish.

There are two version of how the beautiful Nerite was turned into a shellfish.

In the first, the goddess of love herself, Aphrodyte fell for him. Prior to her ascension to Olympian, she lived in the sea where she became close with Nerite. When it came time for her to go, she asked Nerite to join her in Olympus. The only son of Nereus, wishing to stay with his family, refused.  Not used to rejection, Aphrodite offered him wings if he'd come, but again he declined. So she did what any self-respecting love goddess does when spurned--turned him into a shellfish.

The second version, Nerites has again attracted the affection of a powerful god, but this time it is the God of the sea Posedion, and the love is reciprocated. Nerites became of of Posedion's charioteer and his quickness won the admiration of the sea's creatures.  Unfortuntely, his beauty and athleticism also attracted the attention of Helios (personification of the sun). Helios wanted Nerite to be with him, and when his advances were spurned, turned him into a shellfish 

Circles are believed to have protective properties because they lack corners. Corners are considered to be a type of thin place. where energy, supernatural entities, and people might pass through.  By it's very nature therefore, circles serve as the perfect trap.

Celtic folklore: If you want to trap  a druid, put him in a round room.

Celtic Folklore. 
​A term for supernatural thresholds. Thin places are an area where the veil of our reality are well....thin. It is believed energy, spirits, and sometimes people, can pass through these weakened areas and enter the bordering realm.  Thin places can found where two parts meet, such as corners, a fork in a river, crossroads,  and doorways.