Level 2: GPU mining.
Level 3: FPGA mining.
Level 4: ASIC mining.
Around 2,100,000 CIRC is in circulation today (May 3, 2021)
EDIT (November 19, 2020): `getdifficulty` now returns `0.00000236`
EDIT (November 22, 2020): `getdifficulty` now returns `0.00001772`
EDIT (November 24, 2020): `getdifficulty` now returns `0.00002645`
EDIT (November 29, 2020): `getdifficulty` now returns `0.00001744`
EDIT (December 18, 2020): `getdifficulty` now returns `0.00001383`
EDIT (December 20, 2020): `getdifficulty` now returns `0.00001546`
EDIT (December 23, 2020): `getdifficulty` now returns `0.00004681`
EDIT (January 5, 2021): `getdifficulty` now returns `0.00051551`
EDIT (February 4, 2021): `getdifficulty` now returns `0.00035922`
EDIT (May 3, 2021): `getdifficulty` now returns `0.00090437`
EDIT (July 6, 2021): `getdifficulty` now returns `0.00054139`
https://chainz.cryptoid.info/
EDIT (December 18, 2020):
Explorer:
https://chainz.cryptoid.info/circ/
EDIT (December 23, 2020): BTC/CIRC is listed on Bitsails:
https://www.bitsails.com/market?MarketName=BTC-CIRC