First of all, you can't have a metropolis in mountains. Nor can you have a fortification in a metropolis. So the premise is far worse than anything that can actually be achieved in the game.
But try to picture a tank assaulting an urban fortified mountaintop.
If you ask me, 71% is way too high.
If you put the metropolis in hills and acknowledge that you can't build a fortress in a city, the lone tank assault now has an 87.2% chance of success. Bombard with artillery, and your odds go up to 97.9%. Use a veteran tank and now the odds are 99.4%, just as Zach pointed out.
But try to picture a tank assaulting an urban fortified mountaintop.
If you ask me, 71% is way too high.
If you put the metropolis in hills and acknowledge that you can't build a fortress in a city, the lone tank assault now has an 87.2% chance of success. Bombard with artillery, and your odds go up to 97.9%. Use a veteran tank and now the odds are 99.4%, just as Zach pointed out.