A few facts about Kyiv [View all]

it sits on high bluffs above the river Dnipr. Any attack would be uphill

it has the deepest subway system of any major city in the world. The tunnels near the city center are 150-300 feet deep. The defenders can use them as living quarters, shelter and to move from one fighting position to another. Russian artillery can't touch them down there. OTOH it also may tempt Putin to use WMD's to dig them out.