Sanctuaries are the most concentrated places of an aspect, they are bubbles in the ink that cannot harm nor impact reality, they exist outside of time and space. Sanctuaries are linked to some facet of the world their aspect is linked to, for example, Forest Green's (the aspect of life) sanctuary would be connected to forests or other places of great life. However these do not just appear, you need to have knowledge of their existence and a strong enough will to pull on that connection, which is why humans no longer discover these, however they used to be the settings of fairytales and myths, and there are still some stories being found in their very edges.
In the before, the Sanctuaries were the only things to exist, except for the grey plains and the ink. The five realms in the story were Sanctuaries, however, with the request of the aspects, two others, Pink imagination, and Lavender dream constructed a world around them, which would slowly make the five forget their identities as aspects, as dreams do, and provide them with a respite from the complicated world of the before. No one expected them to stay long, however, complicated events led to them being trapped in the dream, but now it isn't quite safe anymore.