Barrier is a mechanic in the series.
Barriers are walls created by Demons that block paths throughout dungeons and Tokyo. These are the mysterious tendrils that compose Domains. There are several varieties of Barriers, but the most common is Wormwood. Only by destroying the core can these barriers be removed. The more powerful varieties require the Jade Dagger and Excalibur to be cut down.
Barriers can permanently manifest if multiple Domains are used to reinforce reality, as is the case of Naraku's lower levels where Medusa once used multiple domains she imposed around herself and her art, and with both her deaths still remain.