What if there is a defensive spell that causes a magic to return to the environment.
Well, in my setting there's no real room for "defensive" magic as such, in terms of shields or combat wards. With sorcerers aspected as they are, you could never defend against every type of sorcery through magic. That said, you could ward an area via a ritual ahead of time, creating a spell that would consume all other magical energy in the area, for instance. Thus any sorcerer coming into the warded area gets shut down until they leave.
Also if you were rich enough, you could use white gold, which when refined properly grounded out magical energy, so sorcery wouldn't work when cast at that person. Of course, their sorcery wouldn't work either. But that only worked against direct attack. It wouldn't stop say, an illusionist from conjuring images to confuse you. Or an evoker could cast at the hillside above you and crush you still. So it stopped direct attacks, but not the natural effect of said attack.
That said, some casters such as illusionists or shadowmages, would by their nature specialize in defensive magic. Shadows for concealment, illusions to pass off yourself as someone else or deceive their senses. So a coven operating together, if they were all sorceresses (not a given, since they're actually very rare in my world among humans) could exercise "combined tactics" as it were to defend one another. Or exercise a ritual ahead of time to prepare the ground in their favor.
