In an effort to trouble shoot this, can those playing on os x post the details of any sieges you've had both those causing crashes and those that did not.
os x 10.11.3
Third age v3.2 (no submods)
* Minas tirith: defending, sally forth
* minas morgul: attacking
* East Osg: attacking, defending, sally forth,
* Cirith ungol: attacking, sallying forth