Milecastles were part of the original plan of Hadrian’s Wall along with turrets and were spaced approximately every Roman mile along the entire stretch of Wall east to west. In effect milecastles were fortified gateways and adapted from the Roman fortlets commonly built in Britain at that time, hence the common term of fortlet for milecastle being coined. Where the Wall was built of stone the milecastles were also built using stone and on the turf Wall they were built from turf and timber. Each one had double gates which were at both the north and south Wall face. Buildings inside the milecastles are assumed to be barracks for the soldiers although one smaller milecastle has had the room attributed to a store room. An oven would be found in the north-west corner usually and in the north-east corner a stairway allowing access to the tower and possibly a walkway on the Wall itself. The tower would be a strong candidate for existence as it would complete the chain of regularly spaced towers along the Wall.
Milecastle dimensions suggest two main sizes for this building type. They were 52 x 12 feet and 20 to 32 x 12 feet with the larger divided into four rooms and the smaller into two rooms. It has been suggested that the milecastle gates were built both at the north and south of the building because of the amount of timbers used to support the towers. At milecastle 50 ten timbers were used at the north gate and six at the south suggesting a more substantial build at the north tower. Six has been offered (C. M. Daniels) as the number required giving the correct support to both north and south towers. The other four timbers may have been for the revetting of side walls of the gate passage. His reckoning comes in that the plans of the gates were the same and therefore only six timbers would be required.
Two types of milecastle were constructed and are known as Long Axis and Short Axis milecastles with the referred axis being that between the northern and southern gateways. Each of the three legions who built the Wall built the gateways in different styles. Gateway Type I was a short axis milecastle built by Legio II Augusta, Gateway Types II and IV were long axis milecastles built by Legio XX Valeria Victrix and Gateway Type III was a long axis milecastle built by Legio VI Victrix.
Type I have piers protruding symmetrically on the inside and outside of the gateway with responds on each. The piers and passage walls tend to be in large masonry. Type II have piers protruding on the inside of the gateway with responds on outside. The piers and passage walls tend to be in smaller masonry than Type I and are only found on Narrow Wall milecastles. Similar gateways found on Broad Wall milecastles are sometimes referred to as Type IV. Type III have piers protruding on the inside of the gateway with responds on both the inside and outside. The piers tend to be in large masonry and the passage walls in smaller material.
There would have been 80 milecastles along the Wall and estimates suggest approximately 20 to 30 sodiers to each. On the northern side of the milecastle a causeway would have been built to cross the ditch which traversed the length of the Wall. J. Collingwood Bruce introduced a numbering system to the milecastles in 1930 and they are simply numbered 1 in the east to 80 in the west.