Let's be honest. You aren't going storm New York City. Rome wasn't sacked until well after the decline.

You need a stronghold. An inside base. Somewhere no one wants to go. Somewhere that already has some existing infrastructure to support life. Somewhere, inside enemy lines that the enemy doesnt even want to be.

The strategy is simple. Make it undesirable to defend. Complacency is your greatest weapon. Make it dangerous. Make it so no one wants to even come near there.

Did the Romans subsidize the Goths?