Quick sitrep:

Kupyansk has 3 commieblock or industrial zones which are great in terms of defence.
Russia is currently storming the largest commieblock area (I think this would qualify as the citadel). The larger industrial area is on the other side of the river.
If Russia can expand control a few more city block south and west then they will pretty much be impossible to kick out for the Ukrainians.

Seversk: Russia is barely a few hundred meters away from the city but are also expanding on the flanks. The pressure is on for real.

Kostyantinovka: Is in a firebag, but nothing critical.

Myrnograd/Pork roast: Only two roads remain, the E50 (within <1000 meters of Russian positions), the northern route is also under Russian fire control, so they have to rely on Ulitsa Olympyska which connects to the northern road. TLDR, they have one small road connecting to a main road left. That's the only road remaining in and out of Myrnograd and Pokrovsk.

Zapo: Ukraine does minor counter attacks and have recaptured two trenches.

ALL OF THESE POSITIONS REQUIRE REINFORCEMENTS and full attention by Ukrainian command or they will fall. Reinforcements that for the moment are preoccupied with counternahruking in Zapo.
But there are 4 mayor defensive positions/losing battles that are all individually eating away at their manpower.

We are very close (probably 3 months) to the tipping point where Ukraine can't defend even their fortezzas.
We see in Pokrovsk how they have to leave gaps in the defences, and we see how they can't send enough troops to save Kupyansk etc.
And their counter attacks are barely company size.

Things move faster now, and they will move faster still a few months from now.