Final answer:
William Shakespeare's play 'Julius Caesar' predominantly draws upon the ancient works of the Greek biographer Plutarch, while also reflecting influences from the broader Roman literary tradition, including the dramatic structures developed by Seneca.
Step-by-step explanation:
The play Julius Caesar is based upon the earlier works of ancient Roman historians, with a notable influence from the Greek biographer Plutarch. Shakespeare drew heavily from Plutarch's Lives of the Noble Greeks and Romans, particularly the life of Julius Caesar and the life of Marcus Antonius (Marc Antony), as a primary source for historical context and characterization in his play. Apart from Plutarch, it is evident that Shakespeare was informed by the Roman tradition of historical writing, including the accounts of other historians who detailed the rise to power of Julius Caesar, and the ensuing political turmoil that would eventually lead to the end of the Roman Republic and the birth of the Roman Empire.
Moreover, the Roman literary tradition, which includes the works of playwrights such as Seneca, who shaped the structure of tragedy in ways that would later be adopted by Renaissance writers like Shakespeare, also influenced the writing of Julius Caesar. Seneca's contributions to drama, such as the five-act structure and the use of supernatural elements, were incorporated into Renaissance and Elizabethan theatre, paving the way for plays such as Julius Caesar to be written.