According to Luke, the author of the book, people helped lower Paul in a basket through an opening in the city wall at night so he could get away safely. They did this because some people were mad at Paul and wanted to hurt him.
Acts 9:25: Then the disciples took him by night, and let [him] down by the wall in a basket.
Paul states that he was let down in a basket through a window in the wall, not specifying it was by his disciples.
2 Corinthians 11:33: And through a window in a basket was I let down by the wall, and escaped his hands.