well for me catapult is a tool, just like i never call a tank as unit, but it might be only me,
As I told you it is a tool but does the tool run it self ?
For your example does the tank drive it self ?
Al this tools catapults and tanks need a human to work so. 1 tank + one human = 1 tool + 1 unit.
The same with catapults it need at least one human to get it to work that means one unit per tool.