Creating a character able to climb while in combat to attack from above

I have played D&D for 8 years now but I’m not that good with character building, so I thought this would be the place to be!

I know that our next adventure will involve a citadel as the main stage of the story. What I would like to make is a character who can climb on walls/ledges, etc. in small alleys or just on the main street walls. My vision is to sneak attack from up high with a bow, or leap down onto the enemy with daggers.

I think it should be some kind of assassin with a bow and good climbing skills. I don’t even know if this is possible or even worth playing.

My group use the all 3.5 books, and the campaign should play from level 1 to about 16–18.