Proto-Indo-European Roots

Root/Stem: *dhwer-, *dhwor-
Meanings:  a doorway, a door, a gate
Cognates: Greek thura (a door) - a feminine a-stem noun
Latin foris (a door), foras (out the house), forum - the initial f- proves it used to be *dh-, not *d-
Umbrian furo, furu (a forum)
Common Celtic *dor- (a door) > 
Insular Celtic *doressuh - a suffix -est- was added; Gaulish doro (a mouth), Welsh & Breton dor, Old Irish dorus, Irish Gaelic doras, Scottish Gaelic dorus or dorust (dialectal), Old Cornish dor, Cornish daras
Common Germanic *dur- (a doorway) with a semi-vowel replacing a root vowel; 
> Gothic dauro (gate), Old English duru (a door), Old High German turi, Old Norse dyrr
Sanskrit dvárah (a door, gate)
Avestan dvar@m (a door), Old Persian duvaraya (outside)
Armenian durkh (acc. a door)
Thracian dur, dero- (a fence) 
> Albanian derë (a gate)
Common Baltic *duaris (gate) > Lithuanian durys (plural for "doors"), Latvian duris, Old Prussian dwaris (gate), Sudovian dvaris (gate)
Common Slavic *dverï (gate) > 
Slovene duri (plural - doors), Chekh dveri, Polish drzwi, Upper Sorbian durje, Lower Sorbian z'urja (doors), Ukrainian dveri, Russian dver'(a door), Belorussian dzwery (doors)
Notes: The noun was feminine and i-stem, though later in some groups (Indic, Greek) it migrated into a-stems. 
Some scientists think (and we share) that the plural form is so frequent because of the meaning "doors": two halfs of the gate. It had dual number, and such a fact is still shown in Russian where the word for "gate" is vorota, the former dual form.