How to use Cast when Damage Taken (CwDT)?

Do the gems need to be at the same level as cwdt or do they need to require the same lvl? They didn't explain it really well.

If CWDT is lv1, then the linked gems must be lv1 (and by level i mean the gemlevel, not required level to use the gem)

Another question that's been troubling me and maybe someone has already tried this:
I've got actually CWDT (lv1) + Enfeeble (lv1); if I link "Increased Duration" (lv15 as an exemple), will CWDT works with it or will ignore that gem?
Is it worth to use Increased Duration?
Thanks for answering

From the patchnotes I am getting the idea that the requirement and not the level of the gem itself has to be lower than the req. of CWDT