Reinforcements

Open and Closed Cities
When asking for reinforcements you should have your troops unhidden (city open) and your highest level hero in the training position in the great hall. If your city is closed, the reinforcements will be defending themselves and depend on the hero sent with the troops. If your highest hero is less than 100, the reinforcements won't get much support.

Same Race and Opposite Race
I've found that when a rein is the opposite race as you, and the incoming attack is the same as you, the rein is attacked first, and doesn't seem to benefit from the city's hero. Reins with the same race as the host are strengthened by the hero on training.

Incoming Race
Incoming attackers will always attack the reins that are opposite race first. References: Basics of Defending/Reinforcing