Although we have a tall and secure side gate for the shared alleyway, I have just purchased some razor spikes (together with a warning sign - although I am not sure whether these type of people deserve any warning) to attach to the top as an alternative to the unsightly razor wire that you sometimes see.

The side alleyways are an obvious weekpoint and it is very easy to use a ladder, sit on the top and pull the ladder over to the other side.

There are some on Ebay and they even cut to size.

There are legal requirements as to where you can situate such measures and you should check them before purchasing.