Mages can use casting circles to temporarily change the magic level of a small area.
When the mage draws a casting circle they can exert their will to pull magic in or out of the surrounded area.
How much a mage can affect the magic level depends on many factors:
The strength of the mage
The mage must exert their will and concentration to cause the effect. Mages with greater will power will have a more pronounced effect.
The mage must maintain concentration to build up the mana. Once the level is obtained the mage must maintain awareness to prevent the magic from "leaking" back out - though they will be able to do other things so long as they maintain conscious awareness of the circle (the mage must stay close and must remain conscious).
How big the casting circle is
A small circle can hold less magic, but the relative difference in magic levels can be greater and the effect will be easier to maintain.
A larger circle can hold more magic, but the magic user will have more trouble keeping it contained so the magic will leak out faster.
A new mage might be able to influence a ring sized circle, while a powerful mage might be able to control a casting circle as big as a room.
How perfect the circle is and what material it is made from
A roughly crafted circle of dirt will be harder for a mage to work with while a finely crafted silver ring might be easy enough for even a novice to affect.
Mages often carry circles of various sizes that they can use to help collect magic if needed - especially when charging tokens or enchantments.
If multiple mages work on the same circle, the effect of their willpower will "average" out so the final effect will be a bit less than the most powerful of them, but on the plus side only one of them has to maintain full concentration to maintain the accumulation of mana - so others can work on building the spell to be cast.
Also, multiple mages can work a larger circle so will be able to have a greater effect that way - but again the effect is not additive. A large group of mages might be able to influence a circle as big as a small house, but it would be extremely exhausting.
Other effects of Casting Circles
Once a casting circle is created and a mage activates it by pushing their will into it, it will act as a kind of "shield" against many magical effects either dampening or completely negating the spell.
Once activated, the casting circle will also prevent fairy activity both inside the circle as well as nearby areas outside the circle.