A Shastargarh, to our knowledge is not a historical Sikh term. However, what we are referring to have been present in history when Saroops of the Guru Granth Sahib were still rare. Before the printing press, Sikh communal places such as Shaheed Ashtaans (memorials for martyrs) would enthrone a collection of weapons as the central point of veneration instead.
I believe that this should once again become a more common place Sikh space as it would not require the same requirements as a traditional Gurudwara/Dharamsala, or the level of observance required that anywhere that houses a Saroop of Guru Granth Sahib should ideally require.
This allows a smaller, more isolated Sangat especially to set up a communal Sikh space with potentially lower costs and commitments, which is useful in the beginning stage of setting up a new Sikh community especially in a new area. As the population of the Sangat builds, attendance increases and funds become more available, then the Sangat could expand in line with the demand. Eventually, if appropriate they could install a Sargun Saroop of Guru Granth Sahib once they have proven they can handle the commitment (such as providing 24/7 security).
There is also a very important community psyche component with consciously bowing to and venerating weapons. The following Vichaars explains a more in depth understanding of the importance of Shastar for Sikhs and the Khalsa in particular.
Additional Vichaars
The Formation of the Shastardhari Panth · Khalsanama