Hi,

use the Gnome Editor and have a look at the Viking build prod file. There you can see that the castle upgrades have these entries (Point 9 - Strategy Map Piece Filename):
"warrior hold, stockade, fortified village, Fort, Fort, Fort, Keep, Keep, Keep"

The info pics for each upgrade are defined this way (Point 8 - Strategy map infopic filename):
"warriors hold, stockade, fortified village, Fort, Fort Motte, Fort Motte Bailey, Keep, Keep Curtainwall, Keep Curtainwall Ballista"

Upgrade: Fort Motte
Strategy Map Piece: Fort
Info Pic: Fort Motte

You'll have to place your bufs in the folder campmap\pieces\buildings.
There must be an entry (or a blank space) for each upgrade in each column (name, costs, build time...) of the build prod file.

Alex