Complex sentence: It had become common practice for sections of the the wall to be leased to burgesses (freemen of the town), which increased the rate of decline of the wall still further and some of the wall was used to support the Glamorganshire Canal embankment.
Split into simpler sentences: It had become common practice for sections of the the wall to be leased to burgesses (freemen of the town), which increased the rate of decline of the wall still further. Some of the wall was used to support the Glamorganshire Canal embankment.