Main building effect = base * level ^ 0.964
Speed effect = base / speed
Natarian village = base / 2 (not in Kingdoms)
Advertisement boost = base * 0.75
Multiply all effects, then round once to the nearest multiple of 10 second.
Most formulas in Travian have one or another type of rounding, but one at most.
May be: Main building effect = base * 0.964 ^ level ?