Hi, the lower caps articles for toponyms is a rule for Catalan language. You can find the description in https://criteria.espais.iec.cat/files/2019/05/3-1-2_Regles-de-maj%C3%BAscules-i-min%C3%BAscules-amb-%C3%ADndex_05.pdf (page 44). Also, see https://publicacions.iec.cat/repository/pdf/00000044/00000018.pdf and notice for example «Torre d'Elna, la». For the Wikipedia, notice that all the uses along the article are in lower caps, except when in the beginning of a phrase. All in all, I would say that the proper form is in lower caps, but I'm open for discussion if you still think otherwise.

Yours,
jmaspons