This stamp issued at the same time as the 1p shows fields of cabbages encompassing a large boulder emerging out of the Sto Plains like a geological pimple, left there by the retreating Ice Giants. The castle is built into this rock. (This may be inspired by Edinburgh Castle.)
The stamps were only available to subscribers of the Stanley Howler Stamp Journal in complete sheets. This may be the reason why a rather obvious variation in this stamp went unrecognized for a while. There were two distinct central images for this stamp. The colour tests were printed what is known as the light sky-tall hill variant. Note the initials AB squeezed into the lower right corner of the cabbage fields. The other print had a redrawn sky and the hill and fields moved down vertically with the height of the hill slightly reduced This became known as the ‘dark sky-low hill’ variety. The light sky variety should perhaps be considered the true stamp. It was this that was used for the colour tests, the resolution of detail of the city on the hill is finer, and it was this image that was later used for the 25p stamp.

 
Comparison of the two hills

The AB initials

The colour test sheet

Imperforate sheet of dark skies