DDR = DC · { L·(L+1)/8 + 5 + (24.875 + 0.625·L2)·L2/2 }
L2 = round{ (L – 1) / 2 }


L — starting level of wall.
DDR — Demolition Damage of Rams, equals percent of worked rams (9).
DC — Durability coefficient, depends on tribe, stonemason, rams' upgrades and durability artifact.