The Porthole Window is a window you can build (the Dusk kit).
In-game description
A moon-birch wall with a round porthole of spirit-glass. After dark it lights the ground outside.SHOWN IN THE BUILD RING
Building it
It costs 3 
Wood, 1 
Ghostcap, 1 Water 1 Earth 1 Fire spirits. It needs 2 Jewels first. Take it down with DISMANTLE to get its cost back (see Building).
