obsidional
Pertaining to a siege.
Nor was this strong geographical position her [Florence's] only bulwark; the capital itself in those unskilful days of obsidional tactics was deemed impregnable except through famine.
Where it comes from
From Middle English obsidional, from Latin obsidiōnālis, from obsidiō (“siege, blockade”), from obsidēre (“beset, besiege, hem in”).
Adapted from the Wiktionary entry obsidional (CC BY-SA 4.0).