This plan of Rome, an exceptional subject for a Book of Hours, is an inset page, painted by the Limbourgs and added to the manuscript as an afterthought. Its place, after the Little Offices and before the Hours of the Passion, is as difficult to explain as its presence. Perhaps it was originally at the head of an Office of Saint Peter and Saint Paul, which disappeared during the manuscript's re-collation in 1485.
It is, of course, tempting to see this strange miniature as a copy of Taddeo di Bartolo's fresco in the Palazzo Pubblico in Siena. But although the two paintings are certainly related, it is doubtful that the Duc de Berry's artists were inspired by the fresco.
Since the Sienese work was painted between 1413 and 1414, at the same time the Limbourgs were at work on the Très Riches Heures, one of the brothers would have had to copy Taddeo di Bartolo's just-completed plan, bring it to Paris, and immediately include it in the Book of Hours.
Furthermore, there are misunderstandings and interpretative errors in the miniature (the sites of the colossal statues on Monte Cavallo and the statue of Marcus Aurelius are left out; the Colosseum is misrepresented) which would not have occurred had the artist seen the fairly explicit fresco.
We must conclude that the Limbourgs based their miniature on a much smaller document, perhaps a mappe-monde (map of the world in two hemispheres) owned by the Duc de Berry according to his inventories.
The most important monuments of Rome are easily recognizable in this figurative plan. At the upper right, outside the city wall, is San Paolo fuori le Mura which, since it was located south of the city, orients the plan. To the left of it, inside the walls, are San Giovanni in Laterano with its campanile, the Temple of Castor and Pollux in pink, and the church of Santa Croce in Gerusalemme in blue.
The right arm of the aqueduct leads to the site of the equestrian statue of Marcus Aurelius (later moved to the Capitoline Hill) and to the Colosseum, represented as a storied tower.
To the right of the Colosseum, a Gothic building with flying buttresses marks the Palatine Hill. Below it is the Arch of Titus, the church of Santa Francesca Romana, and the Capitoline Hill with its campanile; above and to the left is the important pilgrimage church, Santa Maria Maggiore, below which are the Column of Trajan, the Column of Antoninus Pius, and to their right a small round building, the Pantheon.
At the bottom of the miniature, outside the city walls, are from left to right the Ponte Milvio, the Castel Sant'Angelo with its bridge, and beyond the Tiber, a large group of buildings which includes Saint Peter's and the Vatican, above which are the Isola Tiberina and the Trastevere.

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