Hereditary surnames were first introduced into England by some of the leading followers of the Conqueror (William I from 1066 AD), and most were derived from the place-names of their estates either in France or in England. They were usually inherited by the eldest son. The custom of applying a man's to-name to all his children began in the late twelfth century and spread slowly, with the manorial classes and the south of England leading the way. In 1267 is found the first legal recognition of an hereditary surname (de Cantebrigg). By 1400, three-quarters of the population are reckoned to have borne hereditary family names and the process was complete by about 1450. It is these names that make genealogy possible.

Surnames had five main origins: place-names, location of abode, occupations, nicknames and patronymics.

Place-name surnames, derived from towns and villages, indicated either where a person had come from (the majority), or of what place he was lord of the manor or most important resident. In early documents the names were prefaced by 'de' ('from' or 'of'). The large number of 'from' names is evidence of early family mobility.

For a description of the other origins please see 'The Dictionary of Genealogy' by Terrick VH FitzHugh, from which the above was taken. 

'A Dictionary of British Surnames' by  Reany,  Routeledge, Keegan, Paul, 1958. has this to say about the Cruttenden name:
Cruttenden - William Crotynden 1451 PAT (K) A Kent and Sussex name from a lost place Cruttenden in Headcorn (Kent). Pat Calender of Patent Rolls (in progress).

English Placename Society Vol 25 - English Placename Elements has this to say about the derivation of place name endings: 
denn - woodland pasture, Old English, Kent dialect.

Thus originally there might have been a fellow by the name of 'Crutte' who kept his pigs in a clearing in the wood (den). Later when surnames were developing a resident from nearby this woodland pasture who went to live elsewhere may have been known as 'Cruttenden'.

There is another theory that the original place name was derived from a cross in a valley. Local  Old English crücdenu 'Valley with a cross' Kent 15th cent.

I am not convinced that the Cruttenden name is in any way connected with the Druids. In pre-Christian Celtic society they formed an intellectual class. The earliest Classical references to Druids date to the 2nd century B.C.

From the late 15th century there is no doubt that the origin of the Cruttenden name is firmly based in the south-east of England around the borders of Kent and Sussex. That is in one of the first areas of the British Isles to be colonised by the Anglo-Saxons from the 4th century AD onwards. Previous to this there had been over 400 years of Roman rule.

Another theory of origin concerns the clan system in Scotland. As there is no evidence of the use of the Cruttenden surname in Scotland before the arrival of Cruttendens from Kent/Sussex in the mid 19th century this theory must also be discounted.

The Cruttenden surname would appear to be the parent spelling and other spellings are derived from this, such as Crettenden and Crittenden. Crittenden then went on to spawn Crittendon, Critenden, Critendon and Crittenton.