The siege was done quite well, and the engagement in the extended version of the film is one of the best depictions of 18th century warfare in motion picture history;
https://www.youtube.com/watch?v=eBsq6Cd4Rf0
The ambush was completely ahistorical though, as in reality, the English were not allowed to keep their ammunition and were assaulted by the Indians immediately after leaving the Fort(there were even murders within the fort before their departure).
The real event was a completely cowardly and murderous act by the Native Americans who massacred the wounded and straggling Englishmen without much weaponry or ammo, but Hollywood could not portray it as it really happened for obvious reasons.
Now, there is the problem with identifying which tribes participated and which honored the agreement, to my knowledge, this is still unclear.
The ambush in the movie could not have happened, as any army(at the time) will have scouts 100s of meters in every direction, especially when going through a forested area.
The Indians would murder some scouts, gunfire would be heard, and the English would be prepared in the matter of seconds.