Testaccio is our favorite place to stay in Rome and this was our favorite apartment there: https://www.airbnb.com/rooms/2300468?guests=1&adults=1&s=67&unique_share_id=fc3bc7e0-7934-41de-b06c-abc0ccbc341e
Yes, you can just walk to Trastevere. Also easily take the train to Ostia Antica.
That visit was 11 years ago, but I can see from the recent reviews that the apartment and host are as great as ever. Here was my review, which I regret freaked the host out at first:
November 2015......5 Stars
If part of your enjoyment of travel is staying in “adventure apartments” where things leak, blow up, fall apart, won’t open, won’t close, break off, fail to work, or can’t be found --- sorry, you must look elsewhere. Paola’s thoughtfully planned and thoroughly functional apartment will not provide you with any hilarious or horrific stories. However, it is not at all an American-style apartment, but a very Italian one in a very Italian neighborhood. And Paola herself, like her apartment, is welcoming, calm, efficient, organized, easy to understand, and provides you with everything you need. There is even a giant notebook full of useful information in several languages about the apartment and Rome and the Testaccio neighborhood (our favorite area to stay.)
Although I suppose the apartment is technically a studio, it doesn’t feel like one --- it feels so light and spacious and uncluttered that the two of us (who are used to living in twenty times as much indoor space and way more clutter) thought we could easily live in Casa della Robbia for a month or more.
Response from Paola....November 2015
What can I say? Only thanks! I am confused by the accomplishments...I hope to deserve such a kindness. At first, reading the reviews I was very concerned ... oh my God all negative things! Then I realized the joke. Thanks for appreciating Testaccio, I love this neighborhood that has strongly contributed in making my life happy. I hope to see you again in the near future. Please, as soon as you can, give me some news about the conclusion of your trip around Italy! Kisses. Paola