adjective /əbˈsɪdɪənəl/
Pertaining to a siege.
, 1847: Nor was this strong geographical position her only bulwark; the capital itself in those unskilful days of obsidional tactics was deemed impregnable except through famine. — Henry Edward Napier, Florentine History (E Moxon 1847, p. 3)
Wikipedia foundation.