Title text:
They initially thought the containment structure alone would be sufficient, but for good measure they decided to include the Elephant’s Foot inside.
Transcript:
Transcript will show once it’s been added to explainxkcd.com
Source: https://xkcd.com/3316/
What does the elephant’s foot have to do with this siege approach
Xkcd always has another punchline in the alt text, in this one they say they’ll include the elephant’s foot.