Actually, I should think that not having a window would make it *safer*
for the ferrets.  No window to fall out of, or that could suddenly come
slamming down on them.  No screen to get claws stuck in, or to chew
through and escape.  They could get natural light at other times, when
you can supervise them.
 
Take care,
Chana
[Posted in FML issue 4612]