*bhergh- 1

"mound, hill, grave-mound," Old English beorg (West Saxon), berg (Anglian) "barrow, mountain, hill, mound," from Proto-Germanic *bergaz (source also of Middle Dutch berch, Old Saxon, Old High German berg "mountain," Old Frisian berch, birg "mountain, mountainous area," Old Norse bjarg "rock, mountain"), from PIE root *bhergh- (2) "high," with derivatives referring to hills and hill-forts. Obsolete by c. 1400 except in place-names and southwest England dialect; it was revived by modern archaeology. 

In place-names used of small continuously curving hills, smaller than a dun, with the summit typically occupied by a single farmstead or by a village church with the village beside the hill, and also of burial mounds. [Cambridge Dictionary of English Place-Names]

The meaning "mound erected over a grave" was in late Old English. Barrow-wight is recorded by 1869 in Eirikr Magnusson and William Morris's translation of the Icelandic saga of Grettir the Strong. 

c. 1400, "wooden siege tower on wheels" (late 13c. in Anglo-Latin with a sense "bell tower"), from Old North French berfroi "movable siege tower" (Modern French beffroi), from Middle High German bercfrit "protecting shelter," from Proto-Germanic compound *berg-frithu, literally "high place of security," or that which watches over peace." From bergen "to protect" (from PIE root *bhergh- (1) "to hide, protect") or [Watkins] *bergaz "mountain, high place" (from PIE root *bhergh- (2) "high," with derivatives referring to hills and hill-forts) + *frithu- "peace; personal security" (see affray).

The etymological meaning was forgotten, which led to folk-etymologies and a great diversity of spellings. It came to be used for bell towers (mid-15c.), which at first often were detached from church buildings (as the "Leaning Tower" of Pisa and the Campanile on Plaza San Marco in Venice), and the spelling was altered by dissimilation or by association with bell (n.).