Unlike other monsters with similar effects, this one has the defense stat as its limiting condition. This means you can Special Summon high-leveled Zombies with low defense stats. Here are some examples.
Vampire monsters are all Zombie-types and their defense stats rarely go above 2000 even on the high-leveled ones. Because of this Pyramid Turtle can help you summon powerful monsters like Vampire Grace, Vampire Vamp, etc. Special Summoning Vampire Retainer or Familiar can also trigger their effects.