50% of the HP are destroyed or some minimum (around 200) whichever is greater. This means that 4-5 commandos can pretty much take down any city. You'd need infantry to cpature it, but it's the fastest method for capturing cities.
Example: Send 4-5 commandos and 4-5 infantry to an opponents outlying city. Chances are he won't have sufficient defenses there to prevent capturing it. Once you take it - raze structures until he sends reinforcements. This is why you make outposts.
Then attack with main force while he is distracted.