It can be, relatively speaking, quite easy to defend a city, the Syrian rebels in East Aleppo became quite good at it pretty fast, the same goes for the rebels in the Damascus neighbourhood of Jobar (for this latter example I recommend this video [1], it gives a general idea of how it well went; bear in mind that it was filmed from the pov of the government forces).
The problem is that the attacking force at some point realises that the defendants are pretty well dug in and the commanders of those attacking forces also realise that one of the few solutions available in order to achieve victory is to, almost quite literally, flatten the city. That's what happened in East Aleppo (with the help of the Russian airforce), that's what happened in Jobar, too. Not sure if the flattening of Kyiv would be the best thing for the Ukranian people going forward.
The problem is that the attacking force at some point realises that the defendants are pretty well dug in and the commanders of those attacking forces also realise that one of the few solutions available in order to achieve victory is to, almost quite literally, flatten the city. That's what happened in East Aleppo (with the help of the Russian airforce), that's what happened in Jobar, too. Not sure if the flattening of Kyiv would be the best thing for the Ukranian people going forward.
[1] https://www.youtube.com/watch?v=x9ZXPhmR7lQ