This mod is a bit related to the One City Challenge mode. In this case the capital is the only city that can build settlers, the secondary cities will be outposts. These outposts will only build military units and some military related buildings.
In this way it will have less micromanagment than in normal Civ and there will be more war units, concentrating gameplay on warfare and tactics. At the same time your capital can build everything as in the OCC mode, but if the capital of any civ gets capture that civ is destroyed, rest of outposts will be converted to barbarian camps.
Also as outposts will not generate too much income it will be difficult to maintain a big army. It will incentivate the use of military units to raze outposts, pillage or defend continuosly.
Capturing(razing) the capital will destroy the civilization that owns it
Cities cannot be captured they are razed
The AI will have more defenders in the capital city
Secondary cities will act as outposts, with production limited to military units and some military related buildings. They will have limited culture, commerce and research production.
Palace gives 5 culture instead of 1
Cost to buy land increased 5 times
New unit Slave, works similar to engineer helping to rush a bit constructions
You cannot trade cities, neither the AI can
Capitals and Outposts (G&K) 2016-10-05
Capitals and Outposts (G&K)