Reference thresholds (rough) for identifying a dead/abandoned city by Keep level and monarch power, with a 'gray zone' where it's ambiguous: Keep28 dead if <=16M power (gray zone 16M-22M); Keep26 <=10M (gray 10M-14M); Keep24 <=5M (gray 5M-10M); Keep20 <=1.5M (gray 1.5M-2.5M); Keep16 <=500K (gray 500K-2M); Keep12 <=100K (gray 100K-200K). Power correlates with monarch level as well as Keep level, so a low-Keep city with a high monarch level (long playtime, more research/construction) can show deceptively high power while still being effectively dead -- Keep level alone cannot reliably classify power in the gray zone. Reference survey data shows huge variance among 'alive' cities at the same Keep level depending on investment: e.g. Keep35 alive cities ranged from 338M (monarch lvl33) up to 6.9B (monarch lvl35) in the sample.
Original research source: Evony Guide Wiki