Well, back to the main argument, with a much needed correction of my original statement.
Iron grenades did exist in the 1300-1400 time period, having appeared in China in the 12the century. (Gunpowder invented between the 8th and the 10th). However, these were mainly incendiary devices, due to the composition of the gunpowder. (Flamethrower invented in the 10th century). The Real explosive gunpowder was perfected in the mid of the fourteenth century, but didn't arrive in Europe before 1450's. It's unlikely that the Chinese would be willing to give away these secrets, and European chemistry wasn't advanced enough to produce gunpowder anyway. Gunpowder is fairly hard and dangerous to store, and has a tendency of being ruined if it gets wet. Something that's nearly unavoidable during long seatravel, or extended journey by land without proper care.
Most probably, the Sea raiders managed to get their hands on some older, incendiary granades, probably from the islamitic world. Explains why they weren't used to breach the siege. (Stone doesn't burn) and why they were so effective at stopping the siege. (Horses do burn, as does Count Stone).
Edit: Also people. Keep Calm, Carry on, and press reread your posts before posting.
Edit 2: Originally intended to be posted in the main thread, to correct some incorrect statements made earlier. It was locked a short time before the post was finished,so...