The battery most certainly has bulged. I had bulged (and obviously dead) batteries in all my 1xx conquests. To force the door to slide, +1 for the ruler tip above. Then I'd pull!
If no success, you can easily open the PowerBook and remove the top case (three screws under the PowerBook), and that will help move out the battery.
If no success, you can easily open the PowerBook and remove the top case (three screws under the PowerBook), and that will help move out the battery.