The Window Tool is a terrain token that places windows in wall gaps. Windows automatically size themselves to fill the gap they are placed in and can be removed by painting another terrain type in their place or by raising the lower terrain cell up to close the gap.
There is apparently no glass in the window frames, as although they are impassable to players, projectiles will fly right through them. Windows are a good choice to furnish the walls of a settlement, as they decrease the feeling of claustrophobia that some may feel when the walls are solid. They also prevent people from easily making holes in said walls.