Mudtop is an oceanic city near Shoggoth. I believe it is built on top of dirt that was dredged from the ocean floor. It's described as sort of the Mos Eisley of the ocean; a hive of scum and villainy that answers to no authority. It was the location that Bard and Wildcard were originally going to turn in the bounty for Rainbow Dash. Later on, Monket's arrival leaks word that Rainbow Dash is being held on Red Barge, which instigates a large pirate attack to try and capture her.