Advanced settlement structures

The advanced settlement items are unlocked by meeting specific criteria.
 * Picking up Fallout 4 magazines; such as Picket Fences and specific issues of the Wasteland Survival Guide.
 * The Cabot House quest line also unlocks a set of portrait paintings.
 * Being friendly to specific factions unlocks flags.
 * When you unlock the castle as a settlement and clear out the sublevel of it you will be asked to build and test fire an artillery cannon. Upon completion you will be able to build this cannon at settlements.
 * After discovering Bunker Hill you will be sent on a quest to kill raiders and rescue a few traders. Upon completion you will be able to build Caravan Trade Posts in settlements.

Misc
Issue #4 Wasteland Survival Guide offers a new decoration item, the lawn flamingo. Further checking needed for more available items from this Issue.