I have been trying to get this to work all day, and can't see any way to crack it. I know it has been done, but I am damned if I can get the trick of it.

Give the unit all the basic attributes of a siege unit, and the game calls it 'artillery' but won't let it attack walls until you assign it a number of siege weapons. Make this zero, and nothing happens.

How can this be done?

I want a large unit that can smash through walls... not lob rocks at it!