I know this sounds absolutely ridiculous however, I've been thinking of all the crazy ways societies across the world have tried to gain the upper hand on each other with improved weaponry. Figured this would be the best place to raise a question this obscure. Apologies if this has been asked before/ breaks subreddit guidelines (this is my first post ever and I'm really curious/ procrastinating studying for my ochem exam).
The problem with trying this with something like a catapult is that the hard landing would be detrimental to the combat effectiveness of the launched soldier. In principle, one could catapult-launch a soldier with a parachute or hang-glider, but one soldier will be able to do little before the defenders promptly kill him. A catapult that is capable of launching a soldier a useful distance will take quite some time to reload, and the first soldier will be dead (or surrendered) before reinforcements arrive.
Thus, old methods of getting over fortress walls focussed on ladders, siege towers, ramps, etc., which can either deliver more than one soldier at once, or soldiers in quick succession (ladders), without the hard landing. Going through the walls (i.e., breaching them by undermining, artillery, or rams) or under them (tunnelling) were also options.
However, there is one ancient technique for getting over obstacles/barriers that can be considered "launching through the air": pole vaulting. The early history of pole vaulting is mysterious - there is ancient art (Egyptian, and possibly others) that has been interpreted as pole vaulting (but could be pole climbing), and some kind of pole jumping event was included in the ancient Irish Tailteann Games. Pole vaulting only enters history concretely much more recently, in the 16th century, when it was used to cross ditches and streams in the English fenlands (thus, to cross horizontal barriers too wide to jump across, rather than vertical barriers too high to jump over).
In modern times, launching soldiers through the air became an accepted technique. Soldiers would be launched from aircraft, and use either parachutes or gliders to achieve an acceptably soft landing. The classic capture of a fortress by such airborne attack was the capture of Belgium's Fort Eben-Emael by German glider troops in May 1940.
References:
For a very brief summary of the history of pole vaulting, see the "pole vault" entry in Peter Matthews, Historical Dictionary of Track and Field, Scarecrow Press, 2012. For much, much more, see Jan Johnson et al., Illustrated History of the Pole Vault, Sky Jumpers, 2007.
Thank you so much that was really neato to learn about I never even thought about pole vaulting and gliders being use! Now that I think about it single troops being launched sounds like a wildly ineffective way to breach a fortress Hahahaha