Its appears from Greg's preview that siege works as follows:
Setup to Fire: 1 turn
Fire: 1 turn (end-of-turn normally)
There is a high-level promotion that allows them to fire twice (with sufficient movement remaining).
Greg had moved his trebuchet the previous turn and then tried to fire the next turn and was only able to get off one shot since the other movement point was consumed in setting up the siege engine (he didn't catch this).
Also, that trebuchet appeared to take out a rifleman in two shots right after it was promoted - hence it is now Greg's favorite promotion
Setup to Fire: 1 turn
Fire: 1 turn (end-of-turn normally)
There is a high-level promotion that allows them to fire twice (with sufficient movement remaining).
Greg had moved his trebuchet the previous turn and then tried to fire the next turn and was only able to get off one shot since the other movement point was consumed in setting up the siege engine (he didn't catch this).
Also, that trebuchet appeared to take out a rifleman in two shots right after it was promoted - hence it is now Greg's favorite promotion