The style of this artist was to convey the beauty of nature, and also to remind us of the futility of humanity within that. His landscape scenes are breathtaking, covering regions of Germany and Switzerland at their best. He studied and practiced his art for his entire lifetime, repeatedly developing his skills in depicting trees, plants, architecture and mountainous terrain. Even the greatest artistic genius had to practice again and again in order to master their craft - see the drawings of Michelangelo for examples of this.
Landscape paintings are perhaps the most popular choice for art print reproductions because of their wide appeal to the masses - even the most occasional art fan can appreciate a scene of beauty on their wall at home. One does not need an explanation to enjoy this genre of art. Whilst symbolism was used by Friedrich, his work could still attract those who simply liked the technical qualities that he possessed in capturing scenes of the countryside within Germany and Switzerland. Friedrich would spread mist across his scenes and also deliver accurate spreads of forest trees in his scenes, at a time when landscape painting had not been truly accepted by art academics.
The artist produced a huge selection of work during his career but some of the highlights would have to include Wanderer above the Sea of Fog, The Monk by the Sea, The Abbey in the Oakwood, Cross in the Mountains and Chalk Cliffs on Rugen. A quick browse of these works will help you identify the underlying rules around his work, the common themes if you like. Landscapes under different conditions, with additional human involvement was very much his standard route of approach. Friedrich was also involved in many other mediums and materials, particularly in the earlier stages of his career. You might, for example, consider an art print reproduction of one of his many portrait sketches.
Followers of landscape painting more generally would do well to learn more about North European artists. The UK gave us Turner and Constable, France produced Claude Monet and most of the remaining members of the Impressionist movement and in the Netherlands there was a huge number of talented artists who were devoted to painting the beauty of their region, be it windmills or scenes of maritime interest. In fact, they specialised in it to a point of dominating the continent in this genre, over an extended period.