How to increase city strength?

Question

What factors can increase or decrease the combat strength of a city (both standard and ranged strength)?

Known methods,

• Building walls in the city.
• Having a garrisoned unit.
• Applying the Bastions policy card.

If there are expansion specific methods I would be pleased to know those as well.

Answer 1

Between the wiki and a civfanatics post here's what I gathered,

• Era: Each era has a base city strength Ancient being around 10 and Modern being somewhere around 50-60.
• Terrain: Just like in Civilization 5, being constructed on a hill provides additional combat strength for the city. Additionally, settling on a river can increase city combat strength.
• Population: More population means more city strength.
• Buildings: Walls, Castles, Arsenals, and Military Bases all increase the city's combat strength.
• Garrison: A land unit in the city will increase the city's combat strength.
• Districts: Each district contributes to the city's combat strength.
• Palace: The capital's Palace increases city combat strength.
• City-States: If the city is a city-state, the most number of envoys (up to 6) by a single player contributes to the city-state's combat strength.
• Difficulty: The difficulty level you are playing on can also contribute to a city's combat strength.

Answer 2

In addition to the above, the Castellon governor Victor in the Rise and Fall expansion provides

• +5 combat strength once established in a city
• an additional Ranged Strike per turn as a promotion