What did Caesar do to his enemies instead of killing them?

It depends on the enemy. If you are talking about enemies in battle, Caesar was one of the most terrifying leaders on the battlefield. Once a battle was finished, Caesar followed a policy of clemency for most. Although Caesar liked to define it as "mercy" as he often let the defeated keep not only their lives but their money and estates provided they did not take arms against him again. This was just not done on such a scale before Caesar. However, if the enemy had, in Caesar's eyes, proven to be false or had broken a previous oath not to fight, then it was either death or slavery. This is what happened to many Gauls who fought under Vercingetorix. In one siege, when women and children were allowed to leave and instead tried to trick Caear's legions, he had them all either put to death or sold into slavery.