I ran this adventure 4 times at PaizoCon - 1 table did almost no external defense and didn't have any engineers. It was my first table, so I took it a bit easy on them, but it was almost super messy. After that table I became a lot more descriptive of the land. Players also have a hard time leaving Amara Li on her own, she is their captain and they want to defend her. Be sure to reiterate, as many times as necessary, that she is quite capable of taking care of herself and that the need for body guards is only during the meeting with the Way representative.
I've had players put the bear traps in interesting places, like outside different windows that baddies might enter, outside the front and side doors, down the cliff, etc. I had a Druid make animal traps on the sides of the cliff, pour the oil down the murder holes they made and then light it on fire. I've had whale oil end up in some very random places.
I have yet had the time to put in the optional encounter as these battles can take a really long time. Be ready to reward the creative thinking of the players, doing stuff for defense not specifically listed, but would in fact slow down the army, or keep some from arriving.