Ive found out how this works now if anyone else needs to know this info. Its quite simple, delete all of the following

building siege
{
convert_to siege
levels c_ballista_range c_catapult_range c_siege_works
etc

then when youve just got the castle part left you edit the top line building castle_siege to building siege as shown below.

building siege
{
convert_to siege
levels c_ballista_range c_catapult_range c_siege_works

and thats all it is