The creator of this example uses UK spellings; travelled is not incorrect, although in the US we spell it with only one l.
The second sentence is compound and requires a comma after dangerous.
In the example, the second sentence also contains a comma splice. Replace the comma after survivors with a period and begin a new sentence.
They travelled on horseback to the crumbling city walls. It was dangerous, but they had to look for survivors. They needed all the help they could get if they were going to win this war.