Donetsk Airport
Since the beginning of the conflict, Ukrainian government has been instructed to hold the place at all costs.
Mercenaries of the private military companies have arrived there back in March. The first attempt to take it was done by rebels on 26th May, led by former SSU officer Khodakovskiy, "Vostok" battalion troops arrived there armed with only light weapons. As he said it's gonna be easy and quiet, but in the end it turned out to be a trap. Ukrainian troops called an airstrike of Su-25 CAS-fighters and assault helicopters at airport and city of Donetsk.
More than 100 men (both civilians and militias) were killed.
As Ukrainian government troops offensive developed, by the July they've completely deblocked the airport and started gathering there heavy artillery, MLRS, tanks, personnel, fuel and ammo. System of underground communications, old soviet bunkers, underground hangars made it perfect foothold and artillery position.
Tonns of shells have been fired from that place at the city of Donetsk.
Donetsk residents celebrated each artillery strike fired by militias back at the airport at that time, at their modern airport built for Euro 2012 they were so proud of, because Ukrainian govt artillery that caused so much destructions and so many deaths to town has been deployed there.
However the situation changed after government troops offensive was stopped by trapping its groups in numerous "cauldrons".