Anchor every safe to the structure

An unanchored safe can be carried away and opened elsewhere. Anchoring is part of its rated protection.

How

Bolt the safe to a concrete floor or masonry wall using the fixings and holes the maker provides. UK police recommend fitting by a Master Locksmiths Association member.
UL TL-15 requires a safe to weigh at least 750 lb or come with anchoring instructions (US).
Safes.co.uk notes anchoring is especially important for safes under 1,000 kg, and that wall safes rely on correct installation. Germany's VdS says its approved safes under 1,000 kg come prepared for professional anchoring on site, ideally into solid concrete with the supplied anchors.
Avoid fixing into timber stud walls unless extra anchorage is added; the UK Home Office says brick, concrete and masonry are best, and breeze or thermal block is poor for standard fixings.
If fixing to a suspended timber floor, anchor into joists, not floorboards (UK Home Office guidance).
Check the floor can carry the safe's weight. Heavy safes on upper floors may need the engineer to check.

Safety: Safes are very heavy and can crush. Never move one without proper equipment and helpers.