The outdoors are really what make the trullo special and enjoyable! The terrace is very nice, either during the day or in the evening, it's a quiet place, very private.
We unfortunately happened to stay there in September and it rained during one night. We found out the trullo's bedroom was not waterproof and had to leave earlier than planned. The trullo must be great during the summer, as a pied-à-terre if you spend time in the outside, or at the beach, but I don't recommend it during fresher times, as the inside of the trullo is not very cozy.