We don't have synonyms for main guard yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are shin guard, mail guard and main yard.
(n) :
(fortification) The keep of a castle; a guardhouse or building in a fortress where such a body of troops is lodged.
(n) :
(military) A body of troops comprising the chief guard units, especially a unit of cavalry on the wing of a camp towards the enemy, or a group of soldiers in a fortress responsible for dealing with disturbers of the peace.
(n) :
(fortification) The keep of a castle; a guardhouse or building in a fortress where such a body of troops is lodged.
(n) :
(military) A body of troops comprising the chief guard units, especially a unit of cavalry on the wing of a camp towards the enemy, or a group of soldiers in a fortress responsible for dealing with disturbers of the peace.