"Siege mode" refers to a military tactic where a group or army surrounds and blockades a location, such as a city or castle, in order to isolate and capture or conquer it. Full definition
This trailer introduces the all - new Siege mode for Resident Evil 6. (marketplace.xbox.com)
Did anyone else think that the tiny TARDIS in siege mode (great term, right?) (sarahbessey.com)
ZombieSmash has 31 Campaign levels (with the choice of either Normal or Hard mode), an Endless Siege mode and a nifty Sandbox mode, where weapons and Zombies that you've met in the Campaign mode can get friendly in your very own creative / sadistic ways without the risk to your house. (toucharcade.com)