...or how can I destroy that tower?

If an attacking unit gets ON TOP of the tower, can he just attack the tower from within? Provided that the unit has at least one of the LLC's<----Long List of Cualifications from the rule book.

Please answer this for Ranged and CLose Combat attacks.

If I remember right, a figure on top of a tower, or any castle wall, may not attack that peticular part of the castle.

Yup, thats right. You cannot saw away the branch you are sitting on :)

General Stormfox

And considering the fact that your figure would take damage once you destroyed the castle pieces, it is definately not one of the smartest moves anyways.

Poor tower

But I think you can damage a castle piece indirectly even if you are standing on it. Using shockwave against an enemy unit will also damage the wall you are standing on. (you are not attacking the wall directly!!!)