Mods by fktifzobr.

This is indeed a bug; all the icons correspond to the models, but they are assigned to the wrong buildings – the building files simply have the incorrect icons listed.

So, the icon with two levels of walls is actually the icon with gates and two towers because it represents the gates of the second wall.

However, the icon only shows one level of wall, not two. And if you imagine that it's just one level that doesn't look as fortified as the first level on the icons of the Stronghold and Castle, then that's what creates the impression... The bug here is only the mismatch between the model and the icon.