Learn Italian: Ogni torre è visitata da molti turisti. - Every tower is visited by many tourists.

In the passive voice, the past participle acts like an adjective and must agree in gender and number with the subject of the sentence. Since torre (tower) is a feminine singular noun, the past participle visitato changes to visitata.

We usually learn that da means "from", so why is it translated as "by" here?

In passive sentences, the person or thing doing the action (the agent) is always introduced by the preposition da. While da often means "from" or "since" in other contexts, in the passive voice it translates to "by".

Torre ends in -e, and ogni doesn't change its ending. How do we know torre is feminine to make visitata agree?

Words ending in -e in Italian can be either masculine or feminine, and ogni (every) is an invariable word, meaning it never changes its ending to give you a hint. You just have to memorize that la torre is feminine!

Why does turisti end in -i if the singular is turista?