My Lands: Black Gem Hunting

I sent a merchant out to buy some goods at the market - but now the mission icon is red and when I click on it - it says Robbery on the way. It looks like a bunch of orcs or monsters of some sort under the description. Do I have to tell my troops to do anything, and if so how do I? or do they just defend and attack the monsters when they get to my city automatically?

it will be automatic , only thing u can do is :

hide your troop for not them to be killed (that if u think u can't handle the incoming wave)
close door but you will need BG to do so and not worth it (not for a monster attack at least)

maybe i missed something .

I wanted to let you know your advice was right.
it was not very many monsters that ended up attacking my city and I didn't have to do anything - my fortification tower killed them
Thank you for your help