Vault Key Progressive island

"There are a lot of new Mythic monsters coming, you'll need room for them! Get Keys to unlock new slots in your Monster Vault in this Vault Key Challenge. You can already start collecting keys!"

Vault Keys Progressive "Crowded islands? Embark on the Vault Keys Challenge. You'll be able to obtain Keys to open new spaces in your Monster Vault! Make room for new monsters, start collecting now!"

Running out of space in your habitats? The Vault will allow you to store a large number of monsters, so you can call them to your army whenever you need them!

The Monster Vault is a long-term storage solution for monster collectors. With the maximum habitat limit being 90, players can only house a maximum number of 360 monsters in habitats, far fewer than the total number of unique monsters.

Monsters can be directly hatched into it from the Hatchery. Once inside, monsters stop generating gold but can otherwise be used normally in the game without having to be removed.

Progressive Islands-
Progressive islands are events which allow you to obtain a series of rewards. You cannot obtain all rewards at the same time, you have to complete all of the goals, then receive the first prize, complete goals again, then receive then 2nd prize, and so forth.