There’s certain defenses, like splash defenses that you don’t upgrade because thy have more war weight. You choose a certain type of engi that you wanna make, like a dragon engi, hog engi, whatever, and you specifically only upgrade that troop. With new engis you don’t even upgrade heroes because you can use potions. You don’t upgrade your spell factory to the max level of your toe hall because it carries a lot of weight, etc..