Hm... I'll get back to you on traps. My initial thought was to say "They'd cost 25 goods and maybe you'd pay 5 goods every now and then to repair them." But then again, traps are... well, traps. Some kid could wander into the wrong field and fall into a pit. That'd be a real danger in this setting.

I'll think about this and whether or not its viable, or what kind of traps are viable. If you had a forest and good sentries, I'd say it'd be pretty easy, but you're out in the open plains / hills.