Sanctuaries are entities that regularly spawn with numerous shapes surrounding them. Each sanctuary spawns a boss except the Golden Sanctuary, which spawns a Golden Nonagon.
Sanctuaries were added shortly before changelogs. However, they have been buffed and nerfed numerous times. Nests were added to most sanctuaries in 2020, on PATCH 11/14/2019 (2) they were temporarily stopped from spawning bosses upon they're death, the next day after on PATCH 11/15/2019 (1) they were able to spawn bosses on death again.
Golden Sanctuaries are a very strong variant of the egg sanctuary. The sanctuary behaves similarly to the egg sanctuary, but generates a Golden Nonagon when it is destroyed. The Golden Nonagon can evolve into a Golden Icosagon and the Golden Icosagon can spawn Mladic.
Square Sanctuaries produce about 2 dozen giant squares and generate with numerous regular and green squares as well as Quadral Squares, Splitter Squares, Super Splitter Squares, and Summoner Squares. The sanctuary spawns a Summoner upon death.
Triangle Sanctuaries produce about 18 giant triangles and generate with regular and green triangles as well as Sentries, Guardian Lites, Crashers, and Red Runners of various tiers. They generate a Defender upon death.
Snowball Sanctuary like a Egg and Golden Sanctuary. Spawns a Snowflake on death.