["Map of Ancient Rome 271 AD," created by historicair]
[Source: Wikimedia Commons]
["Portrait of the Four Tetrarchs," 4th century]
[Photographed by Nino Barbieri, Source: Wikimedia Commons]
Tetrarchy - Wikipedia
Diocletian - Wikipedia
Maximian - Wikipedia
The Tetrarchy - History of Rome (This is an audio file. The reader/speaker is a little dull, but there's some good information in it.)