Not sure what spolation in the original post means, but I suppose it is
ruining the monuments, right?
My two cents on the matter: I have some experience doing fieldwork in
Portugal and Spain during my PhD. I was looking for amphibians, and the best
place to find them is in small ponds, usually man made, intended to make
potable water available for cattle or to water the crops. Most of these are
in private property. In some cases, it was easy to ask someone in the
vicinity who the field belonged too (usually the exact person we found, or a
close relative), but in other places, there was no one to be found in a 10
km radius. In these cases, we would usually go in anyway (not that I
recommend it) and explain ourselves if someone came by asking what we were
doing. Depending on the height of the fence, the normal reactions would go
from curiosity to threats to call the police (not that anyone did, after we
explain what we were doing or left the premisses). It also helped that we
had a permit from the National Nature Conservation Institute to capture the
animals.
In addition, in Portugal, hunters are permitted to cross fences up to 3
meters without permission in designated hunting areas, during hunting days.
In other words, now your local legislation about trespassing, talk to local
people and land owners and explain clearly what your intentions are. The
national WLM pages should perhaps mention something on the lines off:
"Beware that some monuments are located in private property. Be sure to ask
for permission to land owners before entering private land and taking
photographs."
Gonçalo