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Silhouetted against the evening sky, the tower looks beautiful.
Literal
Evening-sky [against-に] shine-out-[and-て], tower [subject-が] beautiful-[adverbially-く] look.
Literary description. 映える (はえる) means 'to shine, be vivid, stand out' — specifically the way one thing stands out against another background; often used for silhouettes or things illuminated by contrast. 美しく見える uses the [adj-く + 見える] pattern for 'appear X.' 見える ('to be visible, appear') is the spontaneous/potential of 見る. The whole sentence reads like a travelogue or photography caption.