I'm presenting some simulation data here in the interest of helping players calculate what is required to conquer a city full of units backed up by high ambush.
Currently ambush is capped at 70%. So for every 8 units you must kill that have 70% ambush the expected losses solely from ambush is 8*.7=5.6. In other words to kill 8 enemy units you will suffer on average the loss of 5.6 units purely from ambush. Hopefully that isn't any real surprise.
So what does it mean in the context of conquering a city full of units.
Lets say your opponent has a city full of 32 cheap 1 turn Hv Infantry (the quasi-standard defender) with L3 walls (+5) bonus that you want to conquer. You don't have a hero nearby to do the job but do have access to plenty of mediocre units (Rams, Giants, Hv Calv, Mino's etc). How many men will it take to get this job done?
I ran a simulation of 32 Hv Inf in L3 walls vs 7 Giants+1Ram. So the combat numbers look like 32 units of 20 strength vs 7 25 strength giants and 1 20 strength ram. The Attacking stack killed between 7 and 14 men for an average of 10.5 units killed. So it will take at least 3 stacks (killing 11+ each) and at most 5 stacks (only killing 7 each time) stacks to conquer this city.
Now lets say your opponent substitutes in 1 Assassin with 70% ambush into his city full of 31 Hv Infantry (or 1 Assassin + several Ghosts that add up to 70. It makes only a trivial difference in the final numbers). Now how many men will it take to get this job done?
I ran a simulation again with 70% ambush and the same combat numbers (20 strength Hv Inf, 25 strength Giants, 20 strength Ram). The Attacking stack killed between 4 and 8 men for an average of 6 units killed. So it will take at least 4 stacks (killing 8 each) and at most 8 stacks (only killing 4 each time) stacks to conquer this city.
So comparing the two: Without Ambush you need 3-5 stacks (24-40 men, an average of 32) and with Ambush you need 4-8 stacks (32-64 men, an average of 48) or 50% more units to get the job done.
Here's another tip. I ran some other examples using stronger units than Giants. What I found is that it doesn't make much sense to assault with better units against the Ambush because you just lose better units to the Ambush rolls (for example if remove 1 Giant and add Big Red to the stack so all Giants fight at 35, 30 for the Ram and 50 for the RD the kill ratio is only 5-10 or 7.5 units per stack at the cost of 1 RD and it still requires 4-7 stacks to take the city). So unless the enemy city itself has high bonus's your best bet is just gobs of units at a slightly higher strength than the defender and accept the fact you need 50% more units that normal.
KGB
P.S. If you are wondering why if the Avg Ambush kill ratio is 5.6, how come the difference between the stack kill ratio is only 10.5-6=4.5. That's because the theoretical number of 5.6 assumed no kills came from regular combat. But in reality regular combat will kill some of the attacking units and so not quite as many defenders will enter into battle and get their ambush rolls because sometimes the attacking stack is killed off before 8 defenders are killed in this example because the Hv Inf and Giants are close in combat strengths.