I can't reach the tower which Cecil says is a place of power? I just keep going round and round the hill at its base and there are no jump up options, only a jump down option after going through the archway. Is my game broken, there's just no way up to it?

Ha NO , you need to look at the map cecil gave you (it's in your quest items).

If you loke CLOSELY you'll see that you have to make a circle movement, to get to the tower.
You can't acces it by walking around the small patch of land the tower is situated on.

Look at the map.

Check this out. There is an image in the solution post.

http://www.gog.com/en/forum/the_witcher_2/a_matter_of_life_and_death

Ah doh! I looked at the map but I just thought the key info was logged onto my main game map. Silly me, anyway thanks for the help you two.