Avoid the mortar fire in AC4 Black Flag

"Avoid the mortar fire" is a mission objective in Sequence 3 / Memory 06: Proper Defenses

Walkthrough

Pay attention to what areas are yellow highlighted, and avoid them

If you are in a yellow area, sail out of it quickly

When the area turns red, you will take lots of damage which could potentially destroy the Jackdaw and cause desynchronization.

Next objective

Sink the Spanish ships