Freeciv: attack strategy against computer that already has railroad and cannons

I'm having issues attacking a computer that already has cannons and railroad.

My current plan is having a couple of Mechanized Infantry adjacent to an enemy city (for defense of the square) and some Howitzers which I'm using to siege the city (kill the units defending it).

The problem is that a computer with cannons and railroad can reach my units on its territory in one move. I don't have time to build a Fortress there, to improve the defense. And my Mech Infantry is just always overwhelmed by a herd of enemy canons.

What other plans could I use to kill the units in the enemy city? Here are the alternatives I can think of:

• nuke. I really don't want to use nukes. It's so hard to clean the nuclear waste after that.
• a lot of cruise missiles. It will take lots of time to produce and stock a lot of them near the border.
• use Helicopters instead. This is really not my favorite unit, I don't have almost any experience using them.
• use lots of fighters and bombers to bomb the city. This will result in major losses if a city has SAM battery

Any other options which I'm not aware of?

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.

Given that you possess the most advanced land units, and the enemy is less technologically advanced, here are some suggestions:

• Build a road or railroad to the enemy's city. You would have to sacrifice some engineers to build the road, but it will allow your howitzers to attack the enemy city immediately (without any chance of being intercepted by enemy cannons). The disadvantage of this tactic is that the enemy can pillage or block the roads using their units, but this will not be a problem if you possess enough firepower to remove enemy units that are blocking the road.

• Use lots of fighters and bombers to attack the city. However, you could experience significant losses if the city has a SAM battery.

  This is a good idea if you can ensure that the enemy city has no SAM battery. You can find out by sending in a diplomat/spy. If there is no SAM battery, you can safely send in the fighters and bombers. If the enemy city has a SAM battery, you can consider sabotaging the SAM battery using diplomats/spies before sending in the fighters and bombers.