it increases def/res by 30%
It's a 30%reduction to all damage. So for some characters around 30 defense or Resistance that would result in 10 less damage.
Squatting Hector and Nowi on those tiles is pretty awesome. Also great that the enemy AI doesn't take great advantage of them as they can be baited to move out of the tile.
TA3 Xander on a tile in this current red and green meta is great.
Oh..so for units with lower base defense, these tiles are less effective. Hmm.. my Sheena already has 36def&res without any skill, wonder if I can do something to make her practically unkillable on a defense tile.