How to access the cannon outside the castle

The DS version's main feature is a few extra playable characters with special abilities. The Wikipedia article covers most of the functional differences.

However, I would like to bring attention to the main difference in my head: the DS controls are horrible. Since the DS doesn't have an analog stick, you are forced to use either the DPad or the stylus. Both controls are clunkier and more annoying than their 64 counterparts.

I loved the 64 version and got every star in it, and really wanted to play it again with new characters. Unfortunately, the DS controls made me unable to stand playing the DS version for more than a few levels in. It would be nice if they would rereleased the rerelease on the Wii instead so I have some chance of ever completing it. However, if you play it on a 3DS, you can use the circle pad in place of the DPad, which fixes all of those problems. On 3DS, the DS version controls just fine.

A quick glance at a few game guides shows they all seem to give the following approximate advice:

After killing the Big Boo check left and right and you should see small flat areas above the windows, long jump to them. From here either crawl up the slippery slope towards the centre of the roof or long jump over it (crawling is probably safer). Then follow the flat path here to the star.