Koster produced marine subjects and urban views of Dutch cities such as Delft and Amsterdam. His paintings often feature shipping on the IJ or quiet canal scenes. He used a clean, observational style that avoided the dramatic weather common in earlier Romantic painting. Instead, he concentrated on the clear light of the Netherlands and the structural accuracy of sailing vessels. His compositions often include small figures engaged in daily tasks, such as loading carriages or tending to boats at the quay.

Modern collectors value Koster for his ability to document the nineteenth-century Dutch environment. His prints offer a sense of order through balanced horizontal lines and soft atmospheric effects. These works provide a window into a pre-industrial era without relying on sentimentality. The clarity of his architectural studies makes them suitable for contemporary interiors that require a sense of historical perspective.