Just saw this in Make magazine, a performance artist named
Nathan Arnold from Seattle Washington has been doing this for the last few years:
http://johnnyjetpack.com/schematic.htm
Main site:
http://johnnyjetpack.com/
It doesn't quite launch people to the tops of buildings, but does the job for lightweight mannequins.
P.S. If I recall correctly, the usual reponse for being pelted by various bovine projectiles is to construct a large wooden rabbit, roll it up to the castle gates and then have the following conversation: (excerpt from "
The Holy Grail: Scene 8")
ARTHUR: What happens now?
BEDEVERE: Well, now, uh, Launcelot, Galahad, and I, uh, wait until nightfall, and then leap out of the rabbit, taking the French, uh, by surprise. Not only by surprise, but totally unarmed!
ARTHUR: Who leaps out?
BEDEVERE: U-- u-- uh, Launcelot, Galahad, and I, uh, leap out of the rabbit, uh, and uh...
ARTHUR: Ohh.
BEDEVERE: Oh. Um, l-- look, i-- i-- if we built this large wooden badger--
[clank]
[twong]
ARTHUR: Run away!
KNIGHTS: Run away! Run away! Run away! Run away! Run away! Run away! Run away!