The Will of the Senate - out of character thread

[Craterus]

The issue has passed in the in-character thread, but I can't get my head around this. How is Massila easier defended than staying behind the Alps? I see two advantages to leaving it how it is:

1. Irrelevant and incorrect

Spoiler Alert, click show to read: 

It's a neutral (rebel) "buffer zone" between ourselves and the Gauls.

2. There's a HUGE mountain range to separate us from the Gauls. I can't quite understand how a river (this was the argument, correct?) beats a mountain range in terms of natural defenses.

EDIT: A re-read of the Consul reports and the map shows that the Gauls have taken Massilia, but I still do not see how it is easier defended.