Only two of these stations were in operation for the duration of the Earth-Romulan War. The rest of the stations were constructed afterwards. (ENT novel: The Romulan War: To Brave the Storm) Deployed in sector Z-6, they were a mile (1.5 km) beneath the surface of each nickel-iron asteroid in cast rodinium domes. (TOS: "Balance of Terror")
- Star Trek Data File, Vol. 1 states that each station only had 16 personnel, while To Brave the Storm states that each was manned by 60 civilian technical personnel, Starfleet officers and MACO troops.
They were outfitted with the latest sensor and communications equipment of the time. (Star Trek Data File, Vol. 1)
- Presumably, these received technological upgrades over the century between initial construction and the renewed outbreak of hostilities in 2266.
As new outposts were constructed following the war, the sector was patrolled by elements of the Vulcan Defense Force. As well, those outposts would incorporate the hulls and power-generation systems of Vulcan's most potent military vessels. (To Brave the Storm)
In 2266, four of these facilities -- Outposts 2, 3, 4 and 8 -- were destroyed by a lone Romulan Bird-of-Prey testing Federation sensor technology and military strength. The Romulan attacker wielded a plasma weapon previously unencountered by Federation vessels. (TOS: "Balance of Terror")
The four stations destroyed were replaced. 20 years later, Earth Outpost 4 would be the site of a standoff involving Romulan forces vs. the Federation starships Konkordium and Avenger. (Star Trek: Avenger: "By Honor Bound")
By the late 24th century, a total of 23 Earth Outpost Stations would be in service along the coreward Federation/Romulan border. There were also Quebec, Romeo, Sierra and Tango Outposts along the antispinward and rimward Federation/Romulan borders. (Star Trek Star Charts)
The counterparts to the Earth Outpost Stations along the Federation/Klingon boundaries were the Epsilon Monitoring Stations, at least nine of which were built by the 2270s. (Star Trek: The Motion Picture)