。
Lightning struck the tower.
Literal
Thunder/lightning [subject-が] that [attributive-の] tower [to-に] fell.
落ちる takes に for the target it lands on — the tower is marked with に, not を. 雷が落ちる (literally 'thunder falls') is the standard idiom for 'lightning strikes,' since Japanese conceives lightning as descending from the sky to a target rather than as a horizontal flash. The same idiom appears figuratively for a sudden angry outburst.