“is harder to win.” The Hebrew is very difficult and this is one of the generally accepted ways to translate the text (cp. HCSB; KJV; NASB; NET; NIV). However, it is also possible that the Hebrew is more like the translation done by Michael Fox in the Anchor Bible: “An offended brother is like a fortified city, and quarrels are like the bar of a palace.” In that case, if an offended brother is like a fortified city then he would be hard to win over, but another meaning, perhaps even a double meaning, becomes possible: the brother is like a fortified city in that he has shut his brother out; the quarrel has caused him to close and bar his gate and he is not interested in a friendly relationship with his brother any more.
We live in an emotionally undisciplined time when people say very hurtful and often exaggerated or untrue things. This is not only ungodly and sin, it can result in damaging personal relationships for years if not for life. The Bible warns us to put away anger, bitterness, and defaming speech (Eph. 4:31), and it is wise to obey God in that matter. It will make a difference in this life and the next.
“the barred gate of a castle.” The Hebrew is literally, “the bar of a castle,” but that is unclear to most English readers who do not understand that “the bar of a castle” is the bar that goes across the gates and keeps them from being opened. When Samson tore the gates off the Philistine city of Gaza and carried them away, he took them “bar and all” and carried them away (Judg. 16:3). Contentions are like the barred gate of a city; they can make being “open” to each other difficult and even sometimes impossible.