Tower casting a spell

I'm working on a TD as my first ever map, and I'm stuck at a tower that I want to have casting a sleep as autocast,
when the slept target's sleep ticks out it explodes causing XX damage in a YY area.

1. I can't find any conditions of a specific building casting
(should I use a unit and simply make it stationary? how then do I make it buildable)

2. I was thinking of creating a Dummy-unit at "target of attacking unit" that casts sleep, and then when 'last casted' sleep tick out it explodes.
(Having a let's say 100 mana requirement for the spell, and reg that finishes after the explosion has happened, to prevent just sleeping and them letting them walk and alike.)
But my dummy unit never casts the spell I tell it too,
if needed I could show a trigger example of how I usually set it up.

3. What spell should I use to base my auto-cast sleep on?

Auto cast only works when an allied unit is damaged near the caster.
Unless your Td has attacking waves normnal means of autocast wont work.

Attacking is more usually used as a means of "áuto casting" in TD's.
For Conditions ypou can check whenever the tower attacks.

If youre really lost you can always open Skibi's Castle Td and have a look.

Another way would be periodically Picking all units around the tower and then order the tower to cast the spell targeting one of those units..

Use something like curse for the autocast. Whenever a unit is cursed have a dummy spawn and cast sleep on said unit.