Does anyone know how to upgrade strongholds? I've checked the upgrades tab on the phone and the menu you get from inside of the crib and there's nothing in either of those for upgrading the strongholds. Am I missing something?

i think the stronghold itself is the upgrade

@darkvare: If that's the case then I should have the achievement for fully upgrading a stronghold.

There are 3 upgradable strongholds from what I've seen. The Morning Star brothel, the Decker power plant and the Luchadores casino. Once you have unlocked these strongholds through the campaign missions you hit Y/Triangle when you are inside and you select the option to upgrade your stronghold.