These are Landmarks on your Hideout. You need to upgrade each structures to improve the benefits they provide.

Structure Table
Structure Effect Map location Quest Name
Tavern WP
Argo Tavern
The area where you can recruit Pirates and Strengthening items. - -
Pirate's Quarters WP
Pirate Quarter
Increase the  Maximum number of pirates allowed. - -
Training Grounds
Training Ground
You can Strengthen or Promote pirates here. - -
Cannon WP
Increase Duel Stamina. Pirate Island 1-10 To the Open Sea, Smolder's Fleet
Temple WP
Increase Gold Merchant Ship's storage. Fallen Lagoon 2-11 Rift, Tropical Harbor Panga
Windmill WP
Increase Grog Merchant Ship's storage. Aqualess Islands 4-15 End and Beginning of a Story, East Aqualess Sea
Market WP
Produces Gems over time. Fallen Lagoon 2-3 Larissa, Nona Beach
Observsatory WP
Increase Pirates EXP earned each quests. Cocutos Sea 5-16 Strength, Rift's Fleet
Observsatory WP
Hot Spring
Increase Maximum Stamina. Purgatory Sea 3-14 To a New Future, Flame Temple
Merchant Gold WP
Gold Merchanct Ship
Merchant Grog WP
Grog Merchanct Ship
Supplies Gold and Grog periodically. Fallen Lagoon 2-2 Dirk, Tico Channel