freeciv
I find that in Freeciv, when you attack a whole stack of units (irrespective of how large this stack is), if your attacking unit is able to take out one of the strongest units in the stack, the entire stack gets destroyed. I don't understand how this is explained by anything in the CombatRules.
Haven't other people encountered this problem/anomaly? Is this a bug or feature?
Try pillaging the roads? It may be costly, but you can send some cheap units along his roads further up the chain and pillaging some of the squares. It will at least slow him down enough so you can prepare a defence or attack his cannons before they get to you. At the very least, it'll cost a little movement points so his cannons won't be at maximum power.
If you are launching your own game of Freeciv you can play without stack destruction. When you set up the game you can pick your preferred Ruleset Version, and the select More Game Options.
In the Game Settings window navigate to the military tab and deselect the rule Do all units in tile die with defender. Just click apply and you are good to go.
Best Answer
From the wiki page you linked, under Cities and Forts:
This works the same as I remember from the original Civ game, So I'd say this is definitely a feature.