the catapults should be enough to farm lvl 4npcs so put them on as a farm team to "earn their keep". For defense they'll be doing about the same damage as ~1k archers but will have an extra round of fireing or two, on the archers given their range, bad thing is they eat like 5k archers. Dump them if food is too much of an issue and you can't be bothered to use them for lvl4npcs.

The rams are no good on the other hand until you're ready to make tens of thousands of them.