Answering possible question:
Although you will discover that cage is full of dead stinky bodies, it does not impact movement - terrain is not a difficult.
Ceilings up to 30 feet high covered with stalactite, so moving with spider climb of similar feature there would be a difficult terrain.
The only objects one could take cover behind are rocks near the zombie.
One square on the grid is 5 feet
I have not yet decided how to move and measure diagonal distance, will do it approximate for now, in case of uncertain I will try to pick what is best for players. But we will see if it works.
Nat20 - I will double the rolled damage dice for simplicity.