If you guys want to keep to a medieval fantasy standpoint, then there are many examples of things that can work just as well and be period.
One example of which is the medieval Korean H’wacha. This machine, for those that aren’t familiar with it, was a cart that had a vertical box filled with a bunch of honeycomb-like holes. In these holes would be inserted a huge amount of large arrows, each fitted with a small canister of gunpowder. The gunner would light the canister, and send the arrows shooting in the very general direction of the enemy. This weapon was highly inaccurate, but any arrows that hit the men in a formation of enemy troops would definitely not miss it hitting him.
Next on the list was of course the cannon, or bombard. More seen among the Middle Eastern countries than in Europe, though not unheard of before 1400, these were incredibly basic in their structure and construction, and incredibly inefficient. Yet they were deadly when properly used - and when they didn’t blow up in your face. They could hurl huge balls of stone or iron at enemy fortifications, or in this case at enemy Titans, eh? http://www.xenophongroup.com/montjoie/cannon4.gif
Or better yet, instead of destroying your enemy, you could always send them into a cowering rush of (certain-reference-to-Austrian-self-defeat-at-a-certain-town-in-the-early-1700’s-that-cost-10,000-lives) bodies running from your walls with use of fireworks! The Chinese were one of the first ones to discover and use gunpowder, and it was to make fireworks for use in celebrations and to scare the enemy into being nothing more than frightened children. Perhaps the same can be done in this case by frightening away the hordes of enemies attempting to batter down the walls.
Yet, if that is still too passive, then we can always get back to muskets. Very, very early muskets:
If I’m honest I really want muskets but remember they need iron wood and loads of gunpowder also they should have a high chance of exploding if matchlocks and slow reload like really slow. Then there should be cannons which needs load of cast iron and can still blow up in your face are slow reloaders and is slow at moving. Also there should be mortars quicker reload have an arc and fire big balls and can be moved easily
I love mortars and it would be awesome to play with them. However the way you described them would make mortars really over powered compared to muskets or cannons and would detract from the fantasy element of the game as players could just bomb everything into oblivion. Also, unless you were to make a modern mortar with streamlined projectiles, you would still be shooting big lumps of metal, just into the air instead of forward which would need even more black powder and that wouldn’t fit with your initial idea.
Sorry to ‘shoot you down’ but unfortunately it doesn’t really fit all that well
I’m no weapons expert either, and I’m sure somebody will mod it in. I just wanted to say it probably wouldn’t fit into the vanilla game. Also, sorry if it came across as aggressive. Re-reading it, it does seem harsh the way I said it.