Gang strongholds are fortified Blighter bases you raid in Assassin's Creed Syndicate. Clear one out and it turns into a Rook stronghold, chipping away at the Templar grip on that borough; each raid also carries optional constraints for full sync.
Whitechapel: Spitalfields. City of London: Field Lane, Black Swan Yard, Rosemary Lane. Lambeth: Battersea Bellows, Echostreet Alley. Southwark: The Mint, Jacob's Island. The Strand: Clare Market, St. Giles Rookery. Westminster: Devil's Acre, Blue Anchor Alley.
The Blighters were built for Crawford Starrick by Maxwell Roth, who trained a Templar leader for every borough. Starting from Devil's Acre, they spread across London, crushing rival gangs and setting up strongholds as they went. Spitalfields, in Whitechapel, held the last gang still resisting, the Clinkers, who join the twins and become the Rooks.
The stronghold location pages credit Henry Green with pointing the twins at most of these sites. Once a stronghold is cleared, the Rooks move in and use it as their own base.
Spitalfields: free the Clinkers. Battersea Bellows: leader undetected, free Rooks. Echostreet Alley: burn plans, free Rooks. The Mint: stop lookouts, burn plans. Jacob's Island: stop lookouts, leader undetected. Field Lane: leader undetected, burn plans. Black Swan Yard: stop lookouts, leader undetected. Rosemary Lane: free Rooks, leader undetected. Clare Market: burn plans, stop lookouts. St. Giles Rookery: free Rooks. Devil's Acre: free Rooks. Blue Anchor Alley: burn plans, leader undetected.
The Dull Blades upgrade in the gang upgrades menu sabotages the weapon shipments that reach Blighter strongholds, which makes the defenders weaker.
Gang Wars
A gang war is the final fight for each London borough in Assassin's Creed Syndicate: once you have weakened the Blighters enough, their Templar leader accepts a street brawl between the two gangs, and winning it makes the borough Rook territory.
Somewhere That's Green
Somewhere That's Green is the first London memory: as Jacob you step off the train in Whitechapel, chase a pickpocket, race Evie up a factory roof and meet Henry Green, the Assassin who has been watching over the city. It ends with a carriage chase to shake off Blighters.