1013: The private attribute may be used only on class property definitions.

You have an extra curly bracket before removeSelf() method and missing bracket at the end. The class should look like this:

package
{
          import flash.display.MovieClip;
          import flash.events.Event;
  
          public class Caesar extends MovieClip
          {
                    private var velocity:Number;
  
                    public function Caesar()
                    {
                              addEventListener("enterFrame"), move;
                    }
  
                    public function move(e:Event)
                    {
                              this.x = mouseX;
                              this.y = mouseY;
  
                              if (caesar.hitTestObject(MainClip.instance.enemyList))
                              {
                                        trace("caesar geraakt");
                                        removeSelf();
                                        MainClip.instance.finishedMainClip();
                              }
                    }
  
                    private function removeSelf():void
                    {
                              removeEventListener(Event.ENTER_FRAME, move);
                              if (stage.contains(this))
                              {
                                        this.parent.removeChild(this);
                              }
                    }
  
          }
}