Mira,
I thought the same thing but did it yesterday finally after 2 weeks of thinking it was bugged.
Here’s the drill:
You have to time it so you’re there when the harpy attack event is going on— or when the Mardine npc has the gold above his head.
If it’s the harpy event, kill the harpies and close the event.
Then Mardine and his asst. yak a lot about throwing the cows and they throw 3-4—unsuccessfully (poor cows) and then finally the last cow makes it the top of the tower.
THAT is when the cannon gets a green outline around it, and you click to interact.
It then catapults you to the tower, where you’re knocked out for alittle bit, but you’re there—whoo hoo! Try that. ..it should work.