open.
In my opinion, as a plumber, the Ukrainians should be preparing to defend against Russian Troops, not armor, not air, not artillery. Urban warfare is where small bodies of fighters can exert the most power. House to house fighting will turn a hero's legs to jelly. In house to house fighting, neither air, nor armor are effective and it becomes a face to face, person to person struggle to the death. In those conditions, those who are the most dedicated will win over those who are unwilling conscripts who are there by force. Fight the invaders where they are the weakest.
Ukrainians should be reinforcing every basement, they need to be packing away the tools of war in small caches, they need to be determining fields of fire, they need to be setting booby traps, they need to greet the Russian invaders with terror; for behind every closed doors is a threat and any object that they touch could be wired to a bomb. The cities should become graveyards for Russian troops. Let the Russians have the wheat fields, but deny them the ability to control the cities.