According to Acts 19, Paul stayed in Ephesus for well over two years. This chapter recounts some of the memorable events that happened during his tenure there.
After three months of preaching in the synagogue, Paul used up his allotment of goodwill among the Jews. He removed himself from the synagogue to instead preach daily in a lecture hall, where he continued to reach Gentiles for another two years.
Paul's ministry in Ephesus was marked by powerful demonstrations of spiritual power: baptizing in the Holy Spirit leading to an outpouring of the gift of speaking in tongues and prophecy, miraculous healings, and exorcisms. (The story of the seven ill-fated sons of Sceva who tried to copy Paul always makes me chuckle, as well as wonder at the demons' familiarity with Jesus and Paul!)
Regarding the spiritual warfare described in vs. 18-20, it's no wonder that Paul would later write to the Ephesians, "For our struggle is not against flesh and blood, but against the rulers, against the authorities, against the powers of this dark world and against the spiritual forces of evil in the heavenly realms." (Ephesians 6:12)
One thing that always stands out to me is that religious persecution isn't always fueled by a mere difference in belief - often there is a political or economic motive to it. Just as the slave girl's masters in Philippi caused trouble for Paul and Silas when their way of making money from her dried up, so, too, do Demetrius and the silversmiths of Ephesus have an ulterior motive when it comes to rioting against the Christians. Their finances are in peril! No matter what country or century we're talking about, messing with people's money is a surefire way to get into trouble. You cannot serve both God and money.