Complex sentence: Indeed, when his health began failing, he worked from his quarters at the Palace-Monastery-Pantheon of El Escorial that he had built in 1584, a palace built as a monument to Spain's role as a center of the Christian world.
Split into simpler sentences: Indeed, when his health began failing, he worked from his quarters at the Palace-Monastery-Pantheon of El Escorial that he had built in 1584. A palace built as a monument to Spain's role as a center of the Christian world.