Great Antosha
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The icons for the Fortress have incorrect numbers.
This is because the incorrect paths are specified in the buildings themselves – the Fortress uses the Castle icon, the Stronghold uses the Fortress icon, and the Castle uses the Stronghold icon.
By the way, this bug wasn't present in version 1.0; I remember being very surprised when I returned to the game in 2012.
Or maybe this isn't a bug at all? They might have done this because of the appearance of the Fortress and Castle icons, as they didn't want to create an icon with two levels of walls for the Castle.