I started with some mint condition Pirates Eldorado instructions, and stumbled upon a well-priced goldmine! (More info in captions & comments) : r/lego
Lego Building Instructions - Let's Build It Again
BrickLink - Set 6074-1 : LEGO Black Falcon's Fortress [Castle:Black Falcons] - BrickLink Reference Catalog
EVERY LEGO CASTLE SET 1978 - 2016 (LEGO CASTLE HISTORY) - YouTube
LEGO 6080 King's Castle Set Parts Inventory and Instructions - LEGO Reference Guide
LEGO 6085 Black Monarchs Castle Instructions, Castle