Once a building or structure has been de-cursed, you can then restore them. Restoring buildings and other structures will add Happiness.
To restore a de-cursed building or wall, select it and drag bricks into the highlighted spots. More bricks can be obtained from the Mine if you run out.
Fences, streetlamps, stone borders on top of walls, and arches can also be restored. Any of these structures can be restored by tapping to select the structure and then dragging magic onto the cursed structure to restore it.