A key defense mechanism of viruses is their strong outer layer, which is refered to as a capsid. The main role of the capsid is to enclose the genetic material of the virus and help it spread within the population.
The structure of the capsid depends on the type of virus. As an example, Gil-Carton D. et al. (2015) states the role and importance of the disulphide bridges and vertical single B-barrel proteins in Archael viruses. Their order and packaging ultimately helps stabilise the capsid and help perform its job accordingly.