There is a proverb that will serve you well, throughout your entire life. You'll find it in Proverb 25:28 where King Solomon warns us: "He that hath no rule over his own spirit is like a city that is broken down, and without walls." There are some problems with a city that is broken down, and without walls. There are some problems with a soul that is broken down, and without walls.
The first problem is what we let in and what we don't let in. A city without walls is open range. So is a soul without walls. It is my belief that humans have to have some restriction because it's very easy to go off the rails. Without walls, there is no control and no way to keep out evil influence. That is especially true for the person "who has no rule over his own spirit.". When we have no rule over our own spirit, we're like a city with no control and no way to keep out the influence of evil.
In most cases, evil comes in quietly. It doesn't come in like they depict in the old shootim'-up-western shows that most of us grew up with. In my experience, evil creeps in. Quietly.
For proof, let's take Solomon, who wrote most of the book of Proverbs. Although it's recorded that he was the wisest man who ever lived, we find he was far more influenced by people he hung around with than he realized. According to scripture, Solomon did not turn toward other gods...he turned toward his many wives, who turned him toward other gods (1 Kings 11:3).
How would you explain a city without walls? Would it be a place with no restraint? Nothing to stop you? Without walls - you go anywhere you want - at any time you want. Without walls there are no restrictions and no restriction always invites trouble.
The devil is looking for souls without walls or with broken doors. It's up to us to guard the doors of our hearts. We can't leave the door open and let the devil pick off our family, our finances, our peace of mind and our strength.
It was Solomon who advised us to guard the door of our heart (Prov 4: 23-27) and maybe he was speaking from experience. I believe he was telling us we must guard the door of our heart because it's harder to fight the devil when we let him right in our living room. It's easier if he's kept on the outside of the door where he doesn't get a strong foothold.
Reading through Proverbs, we find Soloman warning us, from many different angles, to recognize the influencers around us. Choose influencers who will not "turn your heart toward other gods"
Remember: Soloman didn't choose other gods. He chose the people to be around him who influenced him to turn to other gods.
It is our own best interest to choose our influencers well. Broken walls, leaves broken souls.