The history themed youtube channel Lindybeige has done a 5 episode series specifically on all the things wrong with the movie Ironclad, including an episode about it's representation of siege warfare (episode 5, tactics).

Link to this episode:https://www.youtube.com/watch?v=c4-MkDLJJeA

In principle, there were several problems with the siege warfare in the movie according to Lindybiege, from the positioning of artillery, the inherent problems of an infantery charge of a stone castle, the tactics of the defenders etc.