Cyrodiil (also known as the Heartland) is the center of the Empire (literally). It houses White Gold Tower (also known as the Imperial Palace). White Gold Tower is important because it is the headquarters of the Elder Council and more importantly, the Emperor of Tamriel. The inhabitants of the Heartland are very diverse, but it is the true home of the Imperials. The Elder Scrolls IV: Oblivion takes place in Cyrodiil.
Cyrodiil's climate and environment are quite variable. The Colovian Highlands is a vast, hilly savanna. The city of Chorrol lies on the border of the Colovian Highlands and the Great Forest. The Great Forest is very dense; there little sign of human life but the wildlife such bears run freely here. The closest cities you will find in the Great Forest are Chorrol to the North and the Imperial City to the south in the Nibenay Basin. The Nibenay Basin is very large; follow the basin and it will take you past Bravil and Leyawiin into the vast ocean south of Tamriel. The West Weald is Cyrodiil's countryside. The Weald is very flat, and it's home city is the wine making capital of Cyrodil, Skingrad.
It is believed that the first sophisticated beings to arrive in Cyrodiil were the ancient Ayleids, ancestors to the Dunmer, Altmer, Bosmer, and perhaps the Breton mer-men. The mer we know today are actually the Ayleids who broke away from the main tribe. The Ayleids had a powerful grip on Tamriel. They enslaved the Nords and the Imperials who occupied Cyrodiil. That is until the Slave Queen Alessia led a revolt against the Ayleids, raiding White Gold Tower and ultimately earning their freedom. This marked the long, miserable crumbling of the Ayleid empire.
 Cities of Cyrodill
Bravil - A somewhat rundown town on the west bank of the Niben.
Chorrol - A town located between the luscious Great Forrest and the Colovian Highlands.
Imperial City - The capitol of Cyrodill and the Empire as well as the home to various past emperors.
Leyawiin - A town located along the Niben River where it connects to Topal Bay.