After the Nírnaeth Arnoediad, the sons of Fëanor were driven out of the March of Maedhros and the lands south of it down into Osgiliath where they lived among the Green-elves.

They were there for at least twenty-four years, since the next we hear of them is the attack on Doriath, so I'm assuming that they didn't spend the whole time sleeping under trees, but built some sort of houses, although I think those would be smaller and more rustic than traditional Noldorin architecture, hence the natural wood archway.

The composition of this isn't quite right because I drew Maedhros first just to see if I could get the pose right then had to put Maglor behind him in order to get him to fit the paper.  Probably you are supposed to think about composition *first*.  Oh well.