Austria, officially the Republic of Austria, is a landlocked country in Central Europe, lying in the Eastern Alps. It is a federation of nine states, of which the capital Vienna is the most populous city and state. Austria is bordered by Germany to the northwest, the Czech Republic to the north, Slovakia to the northeast, Hungary to the east, Slovenia and Italy to the south, and Switzerland and Liechtenstein to the west. The country occupies an area of 83,879 km2 (32,386 sq mi) and has a population of around 9 million. The area of today's Austria has been inhabited since at least the Paleolithic period. Around 400 BC, it was inhabited by the Celts and then annexed by the Romans in the late 1st century BC. Christianization in the region began in the 4th and 5th centuries, during the late Roman period, followed by the arrival of numerous Germanic tribes during the Migration Period. Austria, as a unified state, emerged from the remnants of the Eastern and Hungarian March at the end of the first millennium, first as a frontier march of the Holy Roman Empire, it then developed into a Duchy in 1156, and was made an Archduchy in 1453. Being the heartland of the Habsburg monarchy since the late 13th century, Austria was a major imperial power in Central Europe for centuries and from the 16th century, Vienna also served as the Holy Roman Empire's administrative capital. The Holy Roman Empire was dissolved in 1806, two years after Austria had established its own empire, which became a great power and one of the largest states in Europe. The empire's defeat in wars and the loss of territories in the 1860s paved the way for the establishment of Austria-Hungary in 1867.