c'est vrais qu'un trois giga change beaucoup de choses
First, it is possible to reinforce even while the stack attack is on-going. Obviously, its better to have your sentinels see the stack coming and prepare several turns in advance for reinforcements. But sometimes we are caught off guard and need to react quickly. Here are three techniques I've discovered, perhaps others would like to add their own as replies to this thread.
1) The AUTO-MOVE method. Let's say I see several boats approaching a coastal city, and realize they will be attacking in 2 turns. I set up several reinforcers on "auto move" to reach the reinforcement point (usually a city) in 2 moves (or even 3 moves - to handle 2nd wave attacks). Two important issues: (a) be sure you have the "Cancel orders for Friendly combat units" and "Cancel orders for Enemy combat units" preferences unchecked (Ctrl-P), (b) watch at beginning of move and see if units automatically advance - they usually do - but if not press Ctrl-U which forces all automoves to take place at that time. This works even while under stack attack.
2) THE MOVE MORE THAN ONE SQUARE METHOD - Construct a good road network to all cities (ideally, a redundant one in case certain roads are cut). Position your cities close enough to support reinforcement movements. Regular single move units (like warriors) can move 3 squares in one turn when on roads; double move units (like impis) can move 6 squares in one turn when on roads. You might want to situate a warrior or spear on each square of the road to make road blocking/cutting more difficult. Once this is set up, you will find you can use "Go to" or simply drag and drop the unit to the reinforcement location PROVIDED IT IS AT LEAST 2 SQUARES AWAY. This also works even while under stack attack. Note: you will not be able to move units that are only 1 square away during a stack attack; however, there's even a way to do this - see PS at end of this article.
3) THE DOUBLE-MOVE UNIT REINFORCE: Two move unit reinforcers (Horses, jags, e.g.) can even reinforce a city without travelling on roads, provided they are exactly 2 squares away (with no movement penalties, of course)
Note these techniques apply not just to cities, but also to regular places on the map where an enemy might be attacking. For instance when I have a guarded road connecting 2 cities sometimes the enemy will attack the middle of that road hoping to split up my defenses. I will reinforce to the point where he is attacking. The other day, a 6 swords stack attacked the middle of such a road. Initially I had one spear there guarding. When I saw him moving into position, I put in a 2nd spear. From this point, I am golden. He began the stack attack feeling, I'm sure, that he would prevail with 6 swords over 2 spear; however, during the attack I just reinforced that spot with more spears from the two cities the road was connecting. Eventually, all 6 swords died and I lost only 3 spear - at the end both cities and the road were intact.
Hope this information helps you in your defensive game.
Best,
SPM
PS Added a month or two later: If your unit is one square/tile away from the city, you can even reinforce with it, if it has movement left. Select the unit, then press the GoToCity button (CTRL-G) and click on the city name (the one which is one square away).
