On-Map CPs represent mighty strongholds of concentrated power within the land.  All are at least Large Town size.

All on-Map CPs are capable of buying and selling every type of resource and item so long as you roll at least a 'Success' in your Diplomacy Check.

Diplomacy with CPs
SizeBonus
Large Town+6
Small City+8
Large City+12
Metropolis+15

A distinction between On and Off-Map CPs is that: You cannot, normally, permanently alter your attitude status with the OMCPs.  Your presence is nearly anonymous, one more huddled face amongst the rabble.  Therefore your trade and smiling personage won't net you any additional favors.

Each OMCP is associated with two Faction Families.  These families, and thus the CPs they occupy, carry a certain personality with them.  Through longer and more complex Quests it is possible to curry favor with one of these families at the expense of incurring wrath with the other.  It should also be noted, that although these Nobles are exactly as malicious and tax-excising as you might imagine: they will still defend incursions onto their land.  This means that if you wantonly attack/destroy their minor villages and hamlets: the nearby, larger CP will send-out armies to flush you out and kill you.

Havenin: Located in (D4).  The Capital of the Land.  Nearly all trade and decisions flow through here.  The most powerful of the ruling families reside here, ready to exercise the King's will.

Lothar: Located in (K6).  A Large Town, but still important to fishing trade and maintaining in the eastern lands.

Othfay: Located in (H1).  A Small City, a merchant empire of solid repute for the refined luxuries of life.

Moracku: Located in (G9).  A Large City, brimming with militaristic households that rule with an iron fist.  They are ever-ready to prove their valor to the Rulers of the land.

Ur-Hig: Located in (B8).  A Small City, with several magician and spell-wielding families, as well as the best seers in the land.  They function as stalwart advisors.