We find here a combination of artistic genres, with a cityscape, landscape plus figurative work all combined together within a religious scene that continued the artist's focus on the lives of Saints Cosmas and Damian. There may have actually been three further items within the predella, though we have no documentation to actually confirm that commonly-held theory. Dream of the Deacon Justinian remains in a collection in Zurich and that item has been connected to the series of work, plus a further two which no-longer exist. The 15th century art world is full of disagreements and discussions, with many of these issues unlikely to be solved in the future, such is the amount of time that has now passed since these artworks were originally completed. Most historians will refer to Fra Angelico's other paintings and projects to try to fill in some of the gaps of knowledge that exist today, though this cannot always answer these questions confidently.
Several of Fra Angelico's works would depict people being removed from within a city and banished to beyond the perimeter of the wall. Many would then be further punished, including being stoned to death and this was the treatment which led to several martyrs which would then become saints. In the example of Saints Cosmas and Damian Rescued by an Angel we find a happier ending in which an angel comes to the rescue of these two individuals. We see all manner of different activities going on in the background, with the city seen in the far distance. The angel and saints are then identified by the gold paint halos which surround each of their heads and runs in line with how Fra Angelico worked right across his career.
Fra Angelico would become one of the most respected artists from the Italian Renaissance and made some important innovations early in this extensive movement which would then be taken on by others as things continued to develop in later generations. Some have forgotten about the achievements of those earlier artists, but thankfully their reputations are still promoted by others. Alongside this painter, you might also be interested in other great names such as Antonio Canova, Lorenzo Ghiberti and Giorgione who added their own innovations to the various disciplines covered in the Renaissance, Baroque and Neo-Classical eras, with literature, sculpture, architecture and painting all undergoing considerable changes over this period of several centuries. Much of this would happen in Italy before spreading elsewhere but there was also some important moments in Northern Europe as well which are sometimes unfairly underplayed by those with a more limited knowledge of this period.