Forum Romanum
Outlines of Roman History: Reorganization of Empire: The Reign of Constantine
In this entry from William Morey's 1901 textbook you will read about Constantine and the changes he made that had a lasting effect on the Roman Empire.
The History Cat
The History Cat: The Fall of the Roman Empire
Explains some of the reasons the Roman Empire collapsed, how Roman cities and towns fell into ruins, and how barbarian groups took over with there being no strong Roman militia to keep lawlessness in check.