In the rule “Engines”, change

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

to:

There exist two types of Resources that are categorized as Engines: Catapults and Rams. Iron (bar)s are also considered Engines. Engines that are in a Keep can be Activated under other rules.

and

A Catapult must be crewed by exactly two Migrants

to:

A Catapult must be crewed by at least two Migrants

and add the following to the end of the rule:

An Iron (bar) is crewed by exactly one Migrant. When Activated, it reduces the Hit Points of the Infrastructure in the next slot by one.