Pembroke Castle has been closed while an unknown package is investigated by police.
Officers were called to Castle Terrace at around 13:00 BST on Friday after the discovery and the castle was shut for the day.
Dyfed Powys Police said it does not believe there is a risk to the public.
A spokesman said "precautions are being taken" after an item which potentially contained an unknown substance was found.
"There will be a high presence of emergency services as the item is investigated," he added.
The castle tweeted on Saturday morning that it would remain closed until further notice.
***CASTLE NOTICE***— Pembroke Castle (@pembscastle) July 13, 2018
Pembroke Castle will be closed for the rest of today (13 July), and tonight's Ghost Tour has been cancelled. We expect to be open again tomorrow, but please check our Facebook page or... https://t.co/pjFLmf5Wcl
***CASTLE CLOSURE UPDATE*** 14 July 2018— Pembroke Castle (@pembscastle) July 14, 2018
Pembroke Castle will remain closed until further notice. Please check our website or Facebook page for updates before leaving. At this moment we are unsure whether... https://t.co/NPxd8N02GP