Pillars of Eternity

tHe Keep is located in the map of Caed Nuaaccessed through Black meadow

After completing the Act 1 Quest The Old Watcher you will meet the Steward who advises that you become master of the Keep.

Upgrades can easily be accessed through the Radial menu. they simply require coin to start the upgrade and time to pass to finish.
however i would advise a word of caution.

i would recommend building the security upgrades first and dont be too hasty to complete this as another achievement requires you to complete each type of stronghold quest, make sure you complete a minor quest (takes 1 turn) before you progress the prestige upgrades.

While the Keep upgrades simply require time to pass, Stronghold quests are turn based. Each upgrade improves your keeps security or prestige and these affect what type of Stronghold quest is offered to you. The problem is that the achievement for completing each type of stronghold type is based upon turns not time (turns are completed when a quest is completed) most people fail to get the minor quest because by the time they have turned in quests they have improved there keep beyond the minor level which i believe is between 5-15 prestige. So bear this in mind it is complex, i am 40 hours into the game and am just learning the more complex mechanics of the game.