Why Donetsk 'fortress belt' matters so much for Ukraine's defences

"If you look at the topography of the Donbas, eastern Ukraine in general, overall the terrain doesn't really favour the Ukrainians. The city of Donetsk is high ground. It's all downhill as you go west, which isn't great for the Ukrainians in terms of running defensive operations. That's not just about drawing in for the close fight or difficulties going up and down hill, a lot of it is also about observation and thus the ability to co-ordinate artillery fires and other forms of fire support without putting drones up. Likewise bits of high ground are better for radio wave propagation, better for co-ordination of drones.