Coastal villages were generally unprotected throughout the middle ages and monasteries had little or no protection. Whilst the Vikings were bad some of the time, they were more traders than raiders. 'Viking' is a verb, not a noun. To go "a-viking' meant digging shields out from the bottom of the boat and hanging a figurehead on the prow and taking what you wanted.
Far, far worse were the pirates,raiders, slavers of the late middle ages up to the 17th century. Please see the excellent BBC series British Slaves on the Barbary Coast
Where I was born in Devon, England the local coastal village church had a plaque remembering one dreadful Sunday when Barbary coast slaves attacked ,killed or enslaved all of the 180 people who lived there in 1358.
Parts of the Western coast of Spain was partially deserted due to fear of attacks from North Africa and only started to recover at the end of the 19th Century.
Barbary slave trade
As an example of a suffering monastery Lindisfarne is one of the better known. Graham-Campbell, James; David M. Wilson (2001). "Salt-water bandits" London: Frances Lincoln Ltd. ISBN 0-7112-1800-5. The Anglo Saxon chronicles are a useful resource with transliteration.