The yellow building is the Orangerie Palace, built for Landgrave Karl (1654-1730) as a summer residence and place to keep his Mediterranean pot plants in winter. Today it houses a restaurant, a museum for astronomy and physics, and a planetarium. The Aue-Pavillon, a temporary building erected for Documenta 12, faces the Orangerie. You can see a bit of it on the right and more in the next photo.