While farming in the pastoral sense is the exploitation of resource plots outside the city by your troops, another type of farming exists.
When a high level player attacks a much lower level player to steal their stored resources, it is called farming. Without a shield, all of the gathered resources in your city that are not protected by a depot can be stolen if your city is taken by a stronger city.
These are not the resources packaged in inventory. These are the resources that you have obtained from the plots just outside your city walls, from purchases, from opened packages, etc. Resources that have not been gathered from plots are safe from this piracy.
Similar games often have restrictions in place so that you cannot attack another player at a much lower level than you. In this game, a level 30 player can attack a level ten city and loot it. The level ten player has is defending troops killed or wounded and all unprotected resources taken. The only limit is how much the attacking troops can carry off.