Throughout Asia in my travels I found glass shards used to line the top of walls to form a cheap alternative to barbed wire to keep introders out. Glass bottles and window glass are common sources. An entire wall will have this along the ridge, perhaps on several levels. This method was very common several decades ago, but has been eclipsed by strips of metal pikes in many places as they develop. It looks neater than broken glass but I doubt it is more effective. This stuff looks very scary.