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Buildings are structures created in a settlement. They include workshops where characters can use various crafting skills, training halls where characters can learn various martial or magical combat skills, commerce and storage structures such as marketplaces and banks, defensive structures that improve the settlement's guard force or strengthen its defenses against direct attack, and a variety of special-use structures that serve a variety of purposes.
Low-level buildings are relatively cheap to construct, but they offer limited benefits and are easy to destroy. As a settlement invests in improving a building, the building "levels up" and provides better benefits and more expanded capabilities.
Settlements in Pathfinder Online are essentially groups of plots into which buildings are designed to fit. These plots are grouped into several large districts, each with its own potential building sites for buildings of various sizes. Some structures gain a benefit from sharing a district with other structures of similar or linked purpose. Settlement layouts aren't grid puzzles like the game Cathedral; they're more like planned subdivisions with curving roads, some predefined open spaces and lots of various sizes.
The largest and most important buildings can't all be crammed into the same district, but deciding which buildings you want, which districts to put them in, and which plots in the districts to build on will provide settlement leaders with plenty of customization options.