This pretty much boils down to access. If you have the right to access the area, the fact that it is a historical site means little. So if there is a footpath you can walk along the footpath. If it's open access area, common land or right to roam then again no issues. If it's owned then you must obey the laws of the land/owner for example many historical sites in the UK are owned by the National Trust, etc
One major exception to this rule (in regards to historical areas) is
Protection of military war graves
The Protection of Military Remains Act 1986 (1986 c. 35) is an Act of
Parliament in the United Kingdom which provides protection for the
wreckage of military aircraft and designated military vessels.....
The Act provides for two types of protection: protected places and
controlled sites. The primary reason for designation is to protect
the last resting place of UK servicemen (or other nationals).
though access itself is typically accepted:
The Act makes it an offence to interfere with a protected place, to
disturb the site or to remove anything from the site
FYI: this is only UK, I don't believe such a law exists outside of the UK