I'm guessing the gaps are for small animals to get through based on environmental restrictions. So they won't close those up. There needs to be a VERY sharp edge at the top.
FUNFACT: In Nanjing China, neighborhoods with walls around the properties use broken glass bottle pieces mortared into the top of them to discourage thieves from entry.
FUNFACT II: In Nanjing China, thieves use readily available carpet scraps to toss over those bottle shards, and go right on over those walls.