Yoshi IS on top. I've been there. He gives you a 100 credits for speaking to him.
I tried a long time ago to 'climb' the castle walls by jumping up the side but I could never do it. I eventually got to the top using the cannon that opens up after the 120th star is collected. (Note that you CAN climb up the steeple part of the castle though, simply by running up it.)
I tried a long time ago to 'climb' the castle walls by jumping up the side but I could never do it. I eventually got to the top using the cannon that opens up after the 120th star is collected. (Note that you CAN climb up the steeple part of the castle though, simply by running up it.)