Were the St Sernin Church's windows designed to give natural light?
Windows in this era were small and narrow and allowed very little natural light into the interior.
http://www.umich.edu/~eng415/topics/art/art-article.html
http://www.cartage.org.lb/en/themes/arts/architec/middleagesarchitectural/romanesquearchitecture/romanesquearchitectureillustrations/romanesquearchitectureillustrations.htm