循 is in 循环 (xúnhuán, to circulate/cycle), 循序渐进 (xúnxù jiànjìn, step by step), 遵循 (zūnxún, to follow/abide by), and 恶性循环 (èxìng xúnhuán, vicious cycle).
Step radical on the left — a small footstep, the left half of 行. Indexes 循 in the walking-and-following family with 往 go, 待 wait, 徐 slowly. Marks the character as a verb of movement along a path: to follow, to trace, to comply with an established route.
Right side 盾 supplies the sound through a sizeable drift: dùn → xún. 盾 alone pictures a shield raised in defence; here purely sonic, the protective meaning plays no part. The phonetic series is small but its presence cleanly distinguishes 循 from its near-homophone 寻 (xún, to search).
