Korvosa's legal system operates on the principle that every person should have someone with the power to override all their decisions.
The Field Marshall of the Korvosan Guard is appointed by the reigning monarch, but an odious selection can be vetoed by the Noble Houses.
The Seneschal of Castle Korvosa is appointed by the Field Marshall of the Korvosan Guard, but again the Noble Houses hold veto.
The most powerful person in Korvosa is the monarch, who dictates laws and commands the Korvosan Guard to enforce them.
However, the Noble Houses can with a unanimous vote command the Korvosan Guard to return to Citadel Volshyenek, preventing the monarch's dictate from being enforced. And the Sable Company - necessary for joint operations - answers only to the Seneschal of Castle Korvosa.
The Seneschal of Castle Korvosa is the second most powerful position in Korvosa's government.
Kings and queens come and go, sitting the cursed Crimson Throne. When the line of succession is in dispute - or, just as often, when it comes indisputably full stop - the seneschal chooses a new monarch from the city's aristocracy.
Furthermmore: the seneschal has authority to order the Sable Company evict a Queen from Castle Korvosa or place her under house arrest, for any reason or no reason at all.