Mental Zombie

This explained why there were no zombies on the third floor. The entire top floor was full of them. As they approached, they heard their unique roars.

A zombie appeared at the top of the stairs, but it was stopped by the iron fence. The zombie could only stand behind the bars and roar at them. Its hands kept scratching the iron bars, making harsh sounds.

This zombie was actually a level-four zombie!

Su Shanshan was both surprised and confused. How could a level-four zombie be stopped by an iron fence? she wondered.

On closer look, the iron fence was glowing with a faint green light. Is that the mutant dog's superpower? So the zombie is trapped here by the mutant dog downstairs. Why would a level-four zombie be trapped by a level-two mutant dog?

Su Shanshan and Gu Zhao did not understand, but they had to go up.