Aka, How to think outside of the box and get away with it.
Consider the following. The house Reyne of Castamere royally pissed off the Lannisters. They end up fighting and the last of the Reyne retreat into their main fortress known for its subterranean systems, defenses and living areas. The fort is an absolute pain to take and could sustain them for a long time. A siege could prolong the war for far too long, being costly and an assault would likely end up with high casualties and loss for the Lannister Army. Tywin Lannister knew this and ended up deciding on a third option. Close off any and all entries/exits to the fort, leaving only a small opening for the massive amounts of water from a nearby river, which he had diverted into the fort. Flooding and killing every Reyne inside, ending the war and suffering no extra loss in the process.