Ancient celtic forms/words/prefixes can be found inside some of the names of quite a lot cities,rivers,toponyms in general.Here are some:
Ambe- : river (Amiens,FR)
ande- : additive prefix (Angiers,FR)
avallo- : apple (Avallon,FR)
Beal- : river mouth (Bealfast,IRL)
-bona : fundament,village (Bologna(IT),Bonn(GER))
Borvo : "god of thermal springs" (La Bourboule,(FR))
Briga : hill,fortress (Coimbra(PT),Brest,(FR))
cambo- : curve of a water current (Cambridge,UK)
condate : confluence (Conté FR)
cumba - valley
Dubi- : black,brownish (Dublin,IRL)
Dubron- : running water (Dover,UK)
-dunum : fortress (Lyon,Dinard FR,Dundee UK etc) [i think it's the most common]
-durum : fortress,village (Auxerre FR)
Eburo- : the sacred tree of the celts(i can't find the english translation)
-ialo : glade (Auteuil,Creteil FR)
-iano : sacred glade,valley (Mediolanum-Milano IT)
-late : swamp,flats (Arles FR)
-lindo : water,lake (Dublin,IRL)
Magos : market (Nijmegen,NED)
Nanto- : valley (Nantes,FR)