Create a new subrule to the rule “Resources”. Call it “Engines” and give it the following text:

There exist two types of Resources categorized as Engines: Catapults and Rams. Those that are in a Keep can be Activated under other rules.

When an Engine is Activated, the activator chooses Migrants hostile to Keep’s owner in the same location as the Engine to crew it for that activation. If multiple Engines are Activated simultaneously - different iterations of a Labour Action’s effect are not deemed simultaneous - they must use disjoint sets of Migrants as crew.

A Ram cannot be crewed by more than four Migrants. When a Ram is Activated, it reduces the Hit Points of the Infrastructure in the next slot by four less than twice the total Power of its crew, with no effect if the total Power is two or less.

A Catapult must be crewed by exactly two Migrants with total Power 2 or more. When Activated, the activator moves a Stone from the Catapult’s location to another Infrastructure in the same Keep; if this can’t be done, the activation has no effect. That other Infrastructure loses 5 Hit Points, and loses additional Hit Points equal to five times the highest Smarts of the members of its crew.