ancient peace-preservation centers
鎭 centers on stabilizing and securing a place or situation: ancient peace-preservation centers that kept order, and by extension the act of calming, suppressing, or holding something down.
鎭 is the older form of 鎮, combining 金 (metal) with 眞 (which likely contributed sound and the idea of pressing down firmly). The character was used for metal weights or objects that hold things steady, later extended to peacekeeping and suppression.
Metal (金) on the left and a weighty-looking right side (眞) suggest a heavy metal object used to hold things down and keep peace. Imagine an ancient metal seal or weight that stabilizes a region.
For チン, picture a heavy metal chin weight pressing down to keep peace: chin -> チン, and the metal weight holds everything steady.