Celtic *dūno- 'fort'

Celtic *dūno- 'fort' is found as a toponym suffix in areas which were formerly Celtic-speaking as France and Great Britain, where it takes the form -ton. It was also borrowed into Germanic (English town, Dutch duin 'garden', German Zaun 'enclosure').

The origin of this word has been disputed, and some Indo-Europeanists have suggested a possible relationship with Latin fūnus 'burial' (from the unattested meaning of 'burial mound'), thus reconstructing a PIE root *d^huHno-¹. But IMHO this etymology fits better to Germanic *dūnō 'hill, dune' (English down, Dutch duine, borrowed into English dune). I regard this word as a Paleo-European root *d^haunV ~ *d^hūnV 'hill, mound' also reflected in Etruscan θaur(a) 'tomb', with n > r.

A connection with PNC *dompe 'edge, bank', which is the source of Greek táphos (native)  and túmbos (a Pelasgian loanword) 'tomb', is possible, although phonetical developments are not clear. 

From my own experience², I know the meaning 'enclosure' is related to 'thick, dense, packed', so I link the Celtic word to Semitic *duhn- 'fat'.
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¹ See for example Mallory & Adams (2006): The Oxford Introduction to Proto-Indo-European and the Proto-Indo-European World, p. 223.
² For example, Spanish cerrar 'to close', Catalan serrar, French serrer 'to tighten', Basque (Biscayan) zarri 'dense', zarra-tu 'dense; to close', ultimately from PNC *=utɕE(rV) 'thick, fat'.