Bricks of city walls will often be found to contain dates. I saw numbers of them recently immured in the city wall of Kia-ting near Shanghai ; they were marked with the reign of Kia-tsing of the Ming dynasty (A.D. 1522 to 1567) and looked very well preserved. Since that city had been destroyed and burned by the Japanese in 1552, as I learned from a memorial tablet in one of the Kia-ting temples, the mark on these bricks suggest the date of one of the several renovations the wall has undergone in the course of centuries.
Source: wiktionary