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During the Renaissance, Venetian art was at the height of its glory.
Literal
Renaissance-era, Venice [genitive-の] art [topic-は] peak-period [object-を] was-welcoming.
Art history register. 全盛期 ('height of glory,' 'heyday,' 'peak period') is a compound: 全 (full) + 盛 (prosperity) + 期 (period) = 'the period of full prosperity.' ~を迎える is a set expression meaning 'to welcome X,' 'to reach X' — particularly used for reaching a period, stage, or milestone. 全盛期を迎える = 'reach its heyday.' The ~ていました past progressive marks the ongoing state during the time frame. The whole sentence is the kind of thing you'd find in an art-history textbook or museum placard.