A Drush is a very aggressive Strategy that can be done with any Civ, mostly on open maps where it can be hard to wall. Signs to look for that indicates a Drush are A, a early Barrack, usually before 7 minutes. And B, a gold pile lacking 10 or more gold without a mining camp, or seeing a single vil mining gold. The best course of action upon seeing either of these signs, would perferably be to fully wall. But if that can't be done in time, then making a few militias of your own, either fighting them off in the Dark Age going for a Fast Castle, or be first up to Feudal getting the Man-At-Arms upgrade and making a counter attack. If you can't see it coming and is caught off guard seeing them in your base- make some quick walls, for example a few palisade walls around your berries and lumber camps, while getting scouts/archers to fight them off once Feudal.

In Team Games Drushes are less common on open maps, as it's more risky in fear of a pocket striking early with scouts. And is rarely ever seen from a pocket player, unless he by chance gets Celts/Goths and wants to forward while their whole team goes for a aggressive approach.