It's a pleasure to join your ranks!
I build in a wide variety of themes, ranging from official ones to fan-introduced ones.
My first LEGO Castle sets were of the pre-2004 Knights' Kingdom sub-theme, with the Bull and Lion factions.
I also happen to have the Knights' Kingdom-themed LEGO Creator PC game.
My favorite Castle sub-theme, however, is the 2007-2009 Fantasy Era one with the Skeletons and Trolls.