Genesis 4:17

"Cain made love to his wife, and she became pregnant and gave birth to Enoch.  Cain was then building a city, and he named it after his son Enoch.” Genesis 4:17

So if Cain was a restless wanderer how did he build a city?  Back in verse 12, God told Cain that the ground would no longer yield crops for him and he would be a restless wanderer on the earth.  It doesn't sound like this was temporary punishment.  One translator argues that end of verse 12 should actually read, "with dire calamity and fear you will exist upon the earth."  So Cain built the city in an effort to protect himself.  

He named his son and the city Enoch which means "dedicated."  One can only speculate about what he was dedicating his son and this new city to.  Was it God?  Was it safety?  At this point there is no evidence whether Cain continued his life in conflict with God or if he turned to to God for the remainder of his time on Earth.  He talked to God, so he would have a hard time denying the Almighty's existence, but just because you believe something exists doesn't mean you accept it's authority over your life.  Only God knows which Cain choose.