Reinforcing moves your troops from one town to another. This can be useful for defense or just to reallocate troops to a different town.
You can reinforce your allies towns by having them check the "Allows troops of allied members to garrison your current city" box in them Embassy (Only level 1 needed).
NOTE: Troops will use food of whatever city you put them up. Make sure you don't exceed your production in food. NOTE: These troops can only DEFEND. You cannot use the to attack.
- Important to remember in most cases the defending troops that are in the city and the ones in the embassy are stronger then the troops attacking the city. So, 100k archers goes along way in most cases. Although its also important to remember those troops need to be feed and if they die from starving or even getting attacked, you lose your troops.
Reinforce also has other advantages. If your beacon tower is high enough level, you can see exactly when an attack will hit your city. if you can't defend against the attack, before that time, you should click on another city that you have or a valley that you have and click reinforce. Use all your troops with a random hero and bring along as much gold and resources you can(gold is most important since you can't store it in your warehouse). Then, when the enemy leaves, you can recall your hero.