。
She used to often practice violin on the rooftop.
Literal
She [topic-は] rooftop [at-で] often violin [of-の] practice [object-を] was-doing [used-to-ものだ].
~たものだ is the nostalgic past — 'used to do X,' the kind of recollection that comes wrapped in a slight sigh. Distinct from the general-truth ~ものだ ('it's the way of things that...'), this past form specifically reaches back to a habitual past now gone. ヴァイオリン uses the relatively modern katakana ヴ (the U+30F4 character) to spell the foreign 'v' sound; older spellings might use バ instead (バイオリン), and both forms remain in use. Rooftops in Japan, often flat and accessible from school or apartment buildings, have long been a private space for music practice, plant-tending, or laundry.