Something doesn't seem right with this, or I am just very unlucky.
Recently in a game I'm playing, Isabella has attacked one of my cities with 6 conquistadors. Defending the city, I had 1 pikeman with combat 1, 1 longbowman with city defense 1, and 1 maceman with combat 1. Her conquistadors all had combat 1. My city had walls, a castle, and a 20% culture bonus, and all three units were fortified.
So, her conquistadors should have an effective strength of 11.
All of my units should have as a base, 50% from wall + 50% from castle + 20% from culture + 25% from fortification, totalling 145%.
On top of that, my spearman has 100% against mounted + 10% for combat 1, totalling 255%, giving an effective strength of 21.3
My maceman has further bonuses 10% for combat 1, totalling 205%, giving an effective strength of 20.4 (not sure if the 50% vs melee applies to mounted melee units too, so I'm leaving it out)
My longbowman has further bonuses of 25% for being in city + 20% for city defense 1, totalling 190%, giving an effective strength of 17.4
Against a unit with 11 strength (isabella's conquistadors with combat 1) every one of my units does at least 25 damage, meaning they need 4 hits to win.
So, on average, my pikeman will be hit 2.06 times, for a total of 30 damage, leaving it with 70 hp, or an effective strength of 14.7, which is enough for about a 78% chance of victory against a second attack.
On average, my maceman will be hit 2.16 times, for a total of 31.9 damage, leaving it with 68.1 hp, or an effective strength of 13.9, which is enough for about a 75% chance of victory against a second attack.
Finally, my longbowman will be hit 2.53 times on average, for a total of 40.35 points of damage, leaving it with 59.65 hp, or an effective strength of 10.38, leaving it with a mere 35% chance of victory in a second attack.
So what are the chances of Isabella taking my city in one turn (as she did)? about 22% * 25% * 65%, or 3.6%... I guess I'm just really unlucky.