1.you should transform horses into forests as soon as possible. 2.when you have an oil ressource on a swamp field, transform to plains and then to forest for game ressource which produces 4 food and 3 production shields when railroaded. 3. same Deal with gem ressource on djungle Square. 4.There is no way back from a plain/forest square to a grassland/swamp/djungle square. 5. check out the ressources line on the map. Sometimes „mining on an unproductive grassland square may result in a gameOnForest field. Another option is to use Dack‘s Terraform tool to check the yellow spots on the map before transforming a grassland into forest.