(multiplies damage to terrain by 0.5)
In truth the way the game originally worked was it had a set of Armor Multipliers in Castes. The ETs, Tanks, Soldiers each had different Castes they belonged to (13 of them) which applied the damage multipliers.
Caste 0 (Unarmored) was used by terrain.
It would be better to just give Armor Multipliers for Terrain in that case. Applicable to all terrain just to get started.
I made a mod once for TFTD which adjusted the armor multipliers for terrain so that different weapons did altered damage to them. For example I lowered it for Gauss and AP so human weapons wouldnt put holes in walls. Raised it for Explosives (for everyone actually) because I lowered explosion damage across the board (to decrease explosion size). Raised it for Sonic so that way sonic shots would almost always melt walls.
Actually I lowered the numbers all over to make it consistent with smaller explosions, but I raised the damage multipliers in many cases so it was still as effective as before.