For the Catacombs you have to travel on boat with Donkey till you get to a point where you save some fellow ogres.
After that you have to battle some soliders (of course) and then unleash a waterwheel to open the door to the Pied Piper.
He has three attacks, four if you count the soliders but I won't. One is red music symbols (beamed note if you really care what it looks like) come out and follow you and hurt you, just keep moving and you might avoid them. The second is blue and it reveals spiked rollers that go to the middle and go back. The third are yellow and they open cannons which is how you will kill the Pied Piper.
When you see the yellow symbols go to the area it disappeared to and a cannon will appear. It will prompt you to push A when in the right spot and that cannon will injure him. Later it will bring multiple cannons up but I don't think you can get more than one. Will take about 10 cannon hits for him to die.