Buildable monuments as ingame achievement rewards.

Reach certain level of total prestige? Get reward.

Kill certain number of enemies? Get reward.

Farm certain number of plants? Get reward.

Tiered of course. Lets say 5 levels of rewards per aspect of progress.

I was thinking rewards could be just for show monuments you can place somewhere. Special tools/items might work as well. Not sure if they should have function or be passive.

Could be combined with steam achievements.

Where the rewards come from? Out of nowhere? Perhaps another kingdom? Built by one of your own nobles? Not sure.