If I'm not mistaken, RCS starts as the city's ungarrisoned CS; an ungarrisoned city with 11 CS will also have 11 RCS before modifiers. Defense buildings (e.g. Wall and Castle) increase both CS and RCS at once.
Garrisoned units don't increase RCS by themselves. Tradition has a policy that adds a +25% RCS modifier to garrisoned cities.
Military Base and Orders (follower belief building) have a +10% modifier to the city's RCS.
4UC Poland has an UB that adds another +10% RCS modifier to the city.