The special mixture of reality, fantasy, and virtuosity that is particular to early Netherlandish painting is nowhere more apparent than in this exquisite panel. In an episode from the popular legend, Saint George in black Gothic armor pins the dragon to the ground with his lance; at the left kneels the fashionably attired Princess Cleodolinda who was to have been sacrificed to the dragon. George was a Roman soldier living in third-century Cappadocia, but the setting has here been transformed from ancient Asia Minor to the contemporary Belgian countryside.
Passing through a series of overlapping hills, we come upon a walled city surrounded by water and dominated by a castle perched atop a fantastic mountain. This scene is almost certainly imaginary and yet is rendered with the greatest clarity and realism. The attention to specific detail has led to the suggestion that the artist made use of a magnifying glass.
The artist's interest in the depiction of light -- reflecting on George's armor and the dragon's scales -- and atmospheric effects shows the influence of Jan van Eyck. The painting is also stylistically related to manuscript illumination that would suggest this is an early work. The panel may originally have been part of a larger ensemble, perhaps a diptych, and was most likely used for private devotion.
More information on this painting can be found in the Gallery publication Early Netherlandish Painting, which is available as a free PDF https://www.nga.gov/content/dam/ngaweb/research/publications/pdfs/early-netherlandish-painting.pdf