As far as allods go, great mages maintain them, keeping the astral from destroying them, providing the illusion of day and night (can be seen as you travel to any large allod), etc. Yasker is responsible for the whole Imperial Archipelagos. Actually, this is the sole reason why orcs allied with the Empire - they can't have a great mage to protect them. Strina could have become this mage, had she not gone completely insane.
The other thing known (from Coldberg quest line) to protect allods from being dissolved by the astral is meteorite ore. On the bottom of every island you can see meteorite crystals, which leads me to believe that Sarnaut had a layer of meteorite under the crust, which is what protected the planet from being eaten by the astral.
Now, Sarn was encased in this planet and created life only so this life can ultimately destroy his prison. Extrapolation can be made that Sarnaut was originally a ball of meteorite, with Sarn inside it, 12 dragons around it, and Sarn used his power to make it a living planet.
Back to meteorite - for smaller islands, the meteorite left over from the explosion of the world may be enough to protect them, and larger allods need a great mage's protection. Also, meteorite may not be as strong as a great mage's shield, and allows for astral demons to reach the allod, or astral "radiation" to mutate life.