Even though they are too stupid probably for this.. the Ukrainians should have a monster amount of construction crews in Nikolaev with modern tunneling equipment, build a labyrinth underground connecting up defensive positions, and having big bunkers underground for ammo, food, spare parts and connected to the tunnel systems.
Above ground you want to slam in giant concrete structures with numerous floors on them and hardcore thick concrete walls and over engineered. Then of course underground connecting to the tunnels, and like parkades underground you can drive military vehicles down into. And have all these sniper positions and places you can fire mortars from and grenade launchers and RPG's.
The tunnels you don't cheap out on, like actual large tunnels with proper concrete walls and ceilings and all these redundant ventilation systems.
