I think a new defence should be a heal tower. like a healer in a clan castle it heals buildings in a short radius but it heals all the buildings in the area.
stats:
heal: 100
hit points: 200
range: 9 tiles
plus 20 heal per upgrade
plus: 100 hit points
also how about a range expander (someone come up with a name)
it expands the range of all tower within it radius
stats:
expand: plus 2 tile
hit points 200
per upgrade:
expand + 1
hit points + 100
