"HW rulez" mod (add-on) for HoMM3.

9 image boxes with castle pictures (I think arranging them in a column from top to bottom would be convenient, like when making a normal selection). And below each, have labels like "Castle," "Rampart," "Tower," etc., with checkboxes next to each city. Clicking the checkbox would remove the city from the selection. A removed castle could be marked with an "X," for example.

Added after 37 minutes
Another small suggestion: I would like it to be possible to view a hero's guild of mages without a spellbook, without having to buy it. No one prevents you from looking at the items in the store.