CIA World Factbook - information on all countries

Exploring Ancient World Cultures: excellent reference sources for Medieval Europe, the Islamic World, Ancient China, India and the Roman world.

Library of Congress Country Studies - information on all countries.

Lonely Planet : information on all countries, with related data on the history and culture of each.

China
Song dynasty - Thinkquest

Constantinople
Constantinople today

Cordova (Spain)
The rise of the Umayyad dynasty in Spain 

India
Timeline of India