I bring to you a set of Collection Icons, so you can put your medieval CC into labeled folders. They will show up in buy and build mode, when you're creating a new collection.
NOTE: Don't put these icons in your Downloads folder, but here:
In case there is no "Icons" folder, put them in the "Collections" folder.
Start the game and it will generate an Icons folder automatically.
All collection icons belong in there.
Make sure, there are no subfolders in the Icons folder.
If there is a subfolder, take its content out and place it directly in the Icons folder.