Almost lost a city (or three) to a surprise attack by two different enemy civs this past week. I had a pretty good defensive position, the southern half of a triangular peninsula. But my southern cites were only lightly defended. Most cities had a fighter plane, one modern unit and a handful of older outdated units that were due for upgrades. In addition, I had a handful of stealth bombers sprinkled around various cities.

When I went to war on my northern border, I was not really expecting the enemy to counter-attack by sea. In hindsight, I should've expected it as I could not locate as many units as I thought would be defending territory. So they sailed up to my shores and unloaded a large stack of invaders (roughly 6-9 units in the stack).

There was no way that the defenders in my city would be able to dislodge such a beast of a stack. Some of my defenders were still pre-gunpowder (although they got upgraded on the next turn to modern era units with my spare gold).

Instead, I quickly rebased a few bombers to cities surrounding the invasion beachhead. I also started moving modern units (thank goodness I had railroads already laid) in to the surrounding tiles and the city tile to reinforce the city and contain the invasion.

While those reinforcements were getting into place and digging in, my stealth bombers flew multiple attack runs against the enemy stack (losing an aircraft about 1/8th of the time, getting it damaged about 1/3 of the time, and getting away scott-free the rest of the time). By the time my counter-attack was ready, the enemy stack was down to half health for all units and my forces were able to mop them up handily.