Depicts defending city walls (丁) with an axe (戌). 丁 also represents the sound. Based on the original meaning "city", now written as 城. The current meaning is a phonetic loan.
成 and 丁 don't sound similar in modern Mandarin due to historical phonetic changes. They were more similar in older Chinese.成 /*[d]eŋ/, /*m-[d]eŋ/ 丁 /*tˤeŋ/, /*tˤeŋ/, /*tˤreŋ/
Due to historical stylistic changes, this component is less similar to 丁 than it was in ancient scripts.