50-50 chance?
Singleplayer : Archer armies?
-6 defence + 150% (capital + fortified) = 15 defence....not that hard to beat I'd say
->BUT that's only if your late, say after 2000 BC at the earliest
The ai is weak, but we can THINK an use strategy ; if you have a warrior army before 2500 BC knocking at their doors, you take their city : they have no bronze working, and only 1 warrior (rarely veteran, so max defence = 3, and that equals a non-vet army, but you'll have a veteran one, and beat them by 4.5 to 3)
Or you horserush (search for the 'guides' yourself please) and get them even easier and faster.
Waiting untill the late eras is only if you want a fun-win, or a tech/econ/culture win.
Imo it's easier to rush the ai in singleplayer than to cut off your own expansion and get a weaker victory.
Online, it's a different story : you try to get your hands on anny land you can, get the chokepoints fast and annoy your foes as much as possible : even while planning on getting a friendly win, always attack anny free/poorly defended city or settler you see, bring in the occasional spy to sabotage production of a wonder/steal their econ-win gold/ demolish their buildings to keep them from attaking you to often.
Anyways, offline you can do whatever you want, but online, there's a different mentality