I started with the tower in the center (possibly Zenopus' tower) and expanded outward from there. I can draw the faces sort of endlessly (and do so in the margins of papers), and have to remind myself to incorporate other elements. The castle in the center is underwater; note the fish and seaweed around it. There are few hexes in there as well.
For this one I used a new mechanical pencil I picked up recently (nothing fancy, just a Pentel Click 0.7). For others I've gone over pencil with ink, but this time I just scanned the pencil and increased the contrast. It worked well enough, and certainly saved time.
If you are on G+, I was flattered when The Secret DM asked if he could turn this drawing into a map. To this end he had Christian Kessler of the Popular Enchanting blog color it into zones and asked G+ for help with keying it. See here.
See also Monster Face Dungeon and Monster Face Mountain.