Thanks for your comments, they are helpful. I try to summarize:

1. The definition of monument is broad and may differ in different countries. It may, or may not, include archaeological areas or historical sites.

2. We must provide clear warnings to participants that they should not enter private property without asking permission from the owner.

3. In some cases the information about their location may be restricted for security reasons. How to deal with it should be negotiated with the institution responsible.