Go to the library in the castle and get underground. (for more info on this, ask another question.) Then click on the switch. On the other side of the jail bars, the sewer bars go up. Then go to the field where the mill is and use the key (for more info on the key, ask another question) to open the sewer in the ground, which is between the haystacks. then go inside. walk to the left and push the dirt wall. then keep going left. go inside the small hole at the end of the dirt hallway. you are in the jail cell! click on the "invader" and it will explode.