If the game crashes when you end the turn, there can be two different problems:
1) The game crashes as soon as you press the button: This is sadly the worst case as it often indicates a corrupted save. Try to cancel all your building and military production as well as the movements and sieges of all your troops then press the button to end the turn.
If you still crash, please have a look in our PC troubleshooting guide and go through it, especially the disc defragmentation and scan disc parts. Click Here for further PC troubleshooting guidance.
2) The game crashes after pressing the end of turn button while the enemies are playing: We might be able to help in this situation. Please contact us by submitting a support ticket and give us as much detail as possible on the problem in the question.