hmmm i think Darky27 did something like this in his circle defense map.
i think what he did was make an auto cast dummy spell like parasite or something like that without buffs and by triggers make a function in which whenever the tower uses the auto cast spell on something the trigger would add the real spell to the tower, make cast the spell on the position of the unit-target of the auto cast spell, wait a few moments and then remove the ability.
i never really opened to see but thats quite the basic idea GOOD NIGHT!!
well for ensnare, i think it's ez, use web and change eveything to like ensnare, change targets to enemy, ground, air, change model file to ensnare, and blah blah blah, call it ensnare, i didn't try it but i'm sure it will work..