Question: In which year was the Saxon Shore Fort at grid reference [ TQ645047] built?

Context articles: Pevensey Castle
Richborough Castle
Gariannonum
Anderitum
Count of the Saxon Shore
Saxon Shore
Caister Roman Site
Abulci
Walton Castle, Suffolk
Romer

Pevensey Castle is a medieval castle and former Roman Saxon Shore fort at Pevensey in the English county of East Sussex.
 The site is a Scheduled Monument in the care of English Heritage and is open to visitors.
 Built around 290 AD and known to the Romans as "Anderitum", the fort appears to have been the base for a fleet called the "Classis Anderidaensis".
 The reasons for its construction are unclear; long thought to have been part of a Roman defensive system to guard the British and Gallic coasts against Saxon pirates, it has more recently been suggested that "Anderitum" and the other Saxon Shore forts were built by a usurper in an ultimately unsuccessful attempt to prevent Rome from reimposing its control over Britain.
Richborough Castle contains the ruins of a Roman Saxon Shore fort, collectively known as Richborough Fort or Richborough Roman Fort.
 It is situated in Richborough near Sandwich, Kent, in the United Kingdom.
Gariannonum, or Gariannum, was a Saxon Shore fort in Norfolk, England.
 The Notitia Dignitatum, a Roman Army “order of battle” from about AD 400, lists nine forts of the Saxon Shore in south and east England, among which one was called Gariannonor.
 It has been much discussed over the years in terms of spelling ("Gariannonum", "Garianonum", "Gariannum"), purpose (whether it really was intended for defence against Saxon raids), and location (whether Burgh Castle or the Caister-on-Sea site).
Anderitum (also "Anderida" or "Anderidos") was a Saxon Shore fort in the Roman province of Britannia.
 Its remains are located at grid reference [ TQ645047] in eastern Pevensey in the English county of East Sussex and the site was later converted into a medieval castle known as Pevensey Castle.
The Count of the Saxon Shore for Britain (Latin: "comes littoris Saxonici per Britanniam" ) was the head of the Saxon Shore military command of the later Roman Empire.
The Saxon Shore (Latin: "litus Saxonicum" ) was a military command of the late Roman Empire, consisting of a series of fortifications on both sides of the English Channel.
 It was established in the late 3rd century and was led by the "Count of the Saxon Shore".
 In the late 4th century, his functions were limited to Britain, while the fortifications in Gaul were established as separate commands.
 Several Saxon Shore forts survive in east and south-east England.
Caister Roman Site is a Roman Saxon Shore fort, located in Caister-on-Sea, Norfolk.
 It was constructed around AD 200 for a unit of the Roman army and navy and occupied until around 370-390 AD.
 This fort was possibly known as Gariannonum, although the single record that describes it as such may also mean the Roman site at Burgh Castle.
The Abulci were a Roman auxiliary company ("numerus") mentioned in the "Notitia Dignitatum" (ca. 420) as composing part of the garrison of the Saxon Shore Fort of Anderida – the title of one position is listed as "Praepositus numeri Abulcorum, Anderidos" (The commander of the Company of Abulci at Anderida)(Notitia Dignitatum xxviii.20) The unit may have been recruited from the territory of Abula (now Ávila) in Hispania Terraconensis, although a Gaulish origin has also been suggested.
Walton Castle was a Saxon Shore Fort in the Roman province of Britannia.
 The fort was reused by the Normans who used it as the bailey for a castle.
 It stood 30 metres above sea level but was destroyed by coastal erosion in the 18th century.
 It was located in the village of Walton, Suffolk, now part of Felixstowe (grid reference [ TM32253580] ).
A Reference Card or "Romer" is a device for increasing the accuracy when reading a grid reference from a map.
 Made from transparent plastic, paper or other materials, they are also found on most baseplate compasses.
 Essentially, it is a specially marked-out ruler which matches the scale of the map in use.
 The scales are laid out in reverse, such that by lining up the numbers given in the grid reference with the gridlines for the square in question, the corner of the Romer lies on the location whose grid reference you wish to read.
 Some transparent versions have a small hole at the origin when this is not at the corner of the Reference Card.
 This allows access to the map such that the location could be marked with a pencil if using the Reference Card in reverse having been given a grid reference to start with.
 They are used in many types of land navigation and map reading, to give a more accurate grid reference than one just estimated by eye from the grid lines on the map.