Not really a frequently asked question, but I'll answer it anyway.
First, you need to make a new building.
Your best option is to find the building that is closest to the building you have in mind and copy it's lines.
Then change the unit count of the civs from 400 to 401.
Change the unit nr + the 2 ID's of the copied lines to 401.
Then change the name and command language lines.
Change the train button (can range from 1 to 20)
Change the cost
Change the Hitpoints
Possibly change the graphic(stand, dead, walk, damage)
Change the defense stats
Change the construction time
That should be about it for the new building.
Then you'll need to make the building available, by making a tech.
enabling a unit enable watch tower
type: 2 (enable/disable)
unit: 401 (your new building)
class: 1 (enable, 0 would be disable)
attribute: -1
amount: 0
Like that. And if you want units available at the new building, change the train location to 401.
During all this, keep parts 1 (units) and 2(techage) of the extended rord tutorial ready.
Clear?[This message has been edited by Thompsoncs (edited 02-28-2012 @ 05:55 AM).]