。
I can see a tower above the trees.
Literal
Tree [possessive-の] top [at-に] tower [subject-が] is-visible.
木の上 here doesn't mean literally 'on top of a tree' but rather 'above the trees' or 'higher than the trees' — a relative position. The exact reading depends on context; with a tower, 'rising above the treetops' is the natural image. 見える describes the tower's visibility to an unstated observer.