I recommend Hotel Sirene very highly. My wife and I just spent a week there (april 21-28, 2007). The rooms are beautiful, the views spectacular (the Temple of Vesta, the cascade in the Villa Gregoriana, etc.), the common areas tastefully decorated, and the staff very friendly.
Although the hotel does not have its own restaurant (except a lovely breakfast room with a gorgeous view), it is next door to a very good one, Cavallino Rosso.
Unique Quality: The location of the hotel is unique. It stands on a rock peninsula, with the Aniene river gorge on one side and the gorge of the Villa Gregoriana on the other. Also, it is within a 5 minute walk of the center of Tivoli and 10 minutes from the Villa D'Este.
Directions: From the train station, follow the signs to the Villa Gregoriana and then beyond about 400 yards along the same main road. The hotel drive is on the same side as the entrance to Villa Gregoriana and adjacent to the restaurant Cavallino Rosso.
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