Since the patch where the "Frost tower" was fixed, the PDT (Poison Dart Tower) is now chilling and slowing the enemys with it's poison. This happens as soon as the first monster dies and makes the so strong PDT even stronger for the slow is very strong. There was no sphere nor equipment equiped which could provide any freeze on the PDT.
Steps to reproduce:
The PDT does not apply any chilling effect on the monster.
This is a separate issue with the Chilling Strikes skill sphere, it's debuff is being applied by towers.
I can denie it because the chilling strike sphere is not equiped.
I runned different tests with and without it and it had no influence at all. PDT and Sky Guard chilled enemys without it too. It is possible that other towers have this behaviour too.
Oki after testing it with Pandy, the freeze sphere and oil sphere needs to be equiped on the active hero so your own towers benefit from it. So yeah it is the sphere.
Same goes for the monk AntiAir "Sky Guard Tower", they also chill the enemys on hit.