...attack the (reinforcement) settlement-army first. Make sure you pull out stakes, lay out (archers) ambushers to fend of and halt the (main) field-army.
Defending settlements against any army:
When you are sieged by a supirior force, run out the back gate and in the woods. Now hide. The enemy will storm inside the gate, whait in the town center, but nothing will happend. Now, just hit 6X on the time bar, and since there is no count down, you will win after 5 minutes

.
Exceptions in woodless areas or if the enemy got a very fast (cavalry) army.
Generals/siege units will die, becasue they can't hide.