The defending team is usually guarding either a hostage or an explosive, and the attacking team wants to stop them in any way. Siege, or R6S as I'll be calling it, is a deeply complex game centered around the 'breach and clear' gameplay style. The title has included new maps, firearms, and Operators since it's initial release.
It's a game that was built up in the 'Games as a Service' sort of model. Tom Clancy's Rainbow Six Siege has been a consistently popular hit since its release in 2015.
