They noticed that the cabin used by Slippery Trendley was a hundred paces from the main collection of huts and that the woods came to within a tenth part of that distance of its door.

It was heavily built, had no windows and faced the wrong direction.
Moving on, they discovered the storehouse of the enemy, another tempting place.

It was just possible, if a siege became necessary, for several of the attacking force to slip up to it and either destroy it by fire or take it and hold it against all comers.

This suggested a look at the enemy's water supply, which was the river.

A hundred paces separated it from the nearest cabin and any rustler who could cross that zone under the fire of the besiegers would be welcome to his drink.
It was very evident that the rustlers had no thought of defense, thinking, perhaps, that they were immune from attack with such a well covered trail between them and their foes.