For those who don't know, Hamlet Defense was from 1.x.
Essentially, you would help a small village or hamlet get ready for attacks by beastmen.
It was separated into two phases: Preparation and Battle Phase.
During the Preparation phase, which took 50-hours, crafters and gathers could turn in items the hamlet's quartermaster asked for or any kind of materia-enhanced gear, while the fighting classes could Caravan Security missions to help effect the hamlet in positive ways.
Then, during the Battle phase, which took 25-hours, you would go into instanced fights to do things like defeating enemy leaders or protecting the hamlet's supplies. Even crafter and gatherers played their parts, with crafters making weapon from things in the hamlet, while gatherers stole alchemical pots from the beastmen to do various debuffs to them.
FATEs took some of these ideas and rolled with it. >3>