Short answers, pulled from the story.
The European Plain, also called the Great European Plain, is the largest mountain-free landform in Europe. It stretches from the Pyrenees and the French coast of the Bay of Biscay in the west to the Ural Mountains in the east, passing through parts of Belgium, the Netherlands, Germany, Denmark, Poland, and numerous other countries including Russia and Kazakhstan.
Most of the Great European Plain lies below 500 feet, or 152 metres, in elevation. In Western Europe the plain is mostly within 200 miles in width, but it broadens significantly toward its eastern part in Western Russia.
The Loire, Rhine, and Vistula cross the western portion of the plain. To the east, the Northern Dvina and Daugava flow northward, while the Volga, the Don, and the Dnieper drain southward across European Russia.
The plain's large, uninterrupted flatland provides very little geographic protection against invasion. France was invaded through the plain three times after 1870, with two of those invasions successfully occupying the country in 1870 and 1940. Russia and Poland have faced similar vulnerability across their heartlands.
Białowieża Forest is one of the last and largest remnants of the primeval forest that once covered the European Plain. It straddles the border between Belarus and Poland. The plain was largely forested before human settlement and the deforestation that followed.
The European Plain is the most agriculturally productive region in Europe. The European Union's Common Agricultural Policy, which regulates agricultural production, is heavily influenced by the plain. The plain also serves as a major center for trade, industry, and renewable energy production.