On each island, there are a variety of 'magical densities' which roughly translate to the floors of a temple. These influence the strength of creatures, weather phenomenon and magical energies in that area. Typically, the closer to a city, the lower the magical density becomes, this is for two reasons: first, peopple absorb mana into themselves, and secondly, cities were usually established where it was safer.
Every island has a Temple, and the Affinity of that Temple matches the most dominant magic on the island.
Temples are concentrated points of an island's mana, often thought to be the heart of an island. They are not usually 'buildings', rather they are regions where the mana manifests zones with challenges, that offer rewards of increasing value. Each temple offers challenges that regenerate and reconfigure around any group that enters it. Once a group has cleared the 'boss' challenge of a 'difficulty band', they may 'attune' to a temple, provided they are at a 'depth' that exceeds their number of affintiy marks. Doing this adds a mark from that temple, to the person that attuned.
Temples can exist in a variety of forms, such as: A massive sprawlling castle in ruins. A hole in the centre of a lake. A mazelike underground catacomb. A region where time is seemingly frozen.
They can bend the rules of physical space, as they reconfigure around the people 'devling' them.
It is possible to damage a temple, by destroying too much of it's mana, this leaves a scar, and increases the challenge of future 'delvers' near that scar.
Temples manifest gemstones (often from the mana of defeated temple summons), as well as other rewards. Because of this, most temples are controled by local authorities and used to farm gemstones at lesser levels. But, if they are over-farmed (too much is taken from the temple in too short a time) the difficulty increases at prior bands, for example, the first band may become a difficulty of the third, but only reward 1st band rewards.
Temples have a 'depth limit', which is between 10 - 12 depending on the temple - but delving to this point is very challenging.
Delving to a depth is not a 'physical' journey as much as a magical process, so you can't just 'airdrop' into a critical location - though some temples do allow similar puzzle solving.
There is mythology that indicates that the temples are 'sentient' in some way, smart and aware - they are considered to be the island, and as how trees are alive, so too must the island be.