Final answer:
The southernmost point of Caesar's campaigns was Carthage, where he defeated the forces opposing him in the Battle of Thapsus in 46 BCE, securing his power in Africa.
Step-by-step explanation:
The southernmost point of Julius Caesar's campaigns was not Rome, Alexandria, or Cairo, but rather Carthage. Carthage was the stage of critical engagements during the Punic Wars between Rome and the Carthaginians.
Notably, Caesar's activities in North Africa included the pursuit of his rival Pompey during the Civil War, which culminated in the decisive Battle of Thapsus in 46 BCE, south of Carthage in present-day Tunisia. This battle effectively marked the end of opposition to Caesar in Africa and demonstrated his military prowess and ambition as a leader.