Engineer goes on the list. One thing had to be worked out first: a ballista does not fire on the builder's machine, it fires on whoever owns it, so "built by the player" is not something the game can answer when it shoots. Stamping your rank onto the piece as you place it does work, and it means carving the stone later does not silently re-arm everything you already built.

A bit late I know, but

what if the siege weapons and traps themselfes do not change.

Instead they turn friendly if they are in range of a ward, and can't target the players who have access/ are added in said ward. This could make base protection better for a group. Otherwise only the constructor of the machiene is safe and the potential friend might still get shot.