This is about Kingdom Two Crowns.
Islands in Two Crowns are persistent, i.e., they won't reset neither when the Monarch sails away from them nor even if the Crown is lost. Instead of this, time goes by and decay happens slowly.
The decay period is related to the overall day count (the number showing up right on the pause menu screen - see image). If the Monarch loses the crown, an heir will keep up the Kingdom from the first island. The overall day count is reset, and an additional decay, equivalent to one hundred days, is applied to all previously unlocked islands.
Decay still happens after defeating the Greed on an island, confirming that it is caused by time and not (or not only) by the greed.
There are handicaps for the first two islands, making the decay less severe there.
- As soon as the Monarch leaves an island, walls start decaying, being destroyed one by one, from the outermost to the innermost walls, where higher tiers can endure larger number of days. Stumps are left on their spot, as if they were demolished by the Greed.
- When all walls on at least one side of the town have decayed, subjects start losing their tools, remaining as villagers and wandering around the Kingdom.
It does not affect:
- Villagers: no villagers will lose their coin.
- Towers: no towers will be destroyed, but archers won't be on top of them. As soon as the Monarch arrives again on an island, if there are any hunter archers available, they will prioritize filling all towers slots. On late game, uncovered towers can be in a vunerable situation if they lose the support of ground troops after some of the outer-walls are destroyed.
- Farms: farms will be intact, but with no crop fields around them. A mill house located on a side with active greed portal in a sector whose walls were destroyed by the decay, will expose the farmers, as they will work on that unprotected place and try to stay there over night.
- Banker: the amount of coins left whith the banker does not change.
The more protected you let an island when you leave, by having a lot of strong walls, the best it will resist decay while you'll be away. An iron wall can survive thirty days.
If the Monarch intends to pass a huge amount of days away from an island, it's probably more useful to make generous deposits at the bank—so new tools can be quickly bought for subjects when returning there—rather than to build strong walls on every single dirt mound (which also takes time and coins).
Reviving a land after two hundred days of decay, grants the "Their Savior" achievement.