As I expect would happen to most groups, Gogmurt sent his little bird to warn Ripnugget.
So, the door to the stockade is barred.
Is there any way to get in besides getting an axe and bashing in the door?
Presumably anyone chopping at the door is going to end up fighting the entire first level of Thistletop at once?
I am GMing and want advice on what options the PCs have?
C13 or C15. As the GM, you can choose how alert sentries are. A well placed Sleep spell from a good range can fix a watchtower. Hypnotic Pattern, Fog Cloud, Minor/Silent Image (to name a few) can create distractions as the group closes in and prepares to climb into a tower, etc. Summon Monster/Nature's Ally to attack tower sentries (not sure about range but there's some foliage cover on the map next to the towers iirc.).
Hope your group gets rewarded for some creativity (subtle hint). :)
Late Edit*** IIRC, weren't some Goblins trying to hit birds or something similar? Imagine using a Summon II spell for 1-3 Eagles to attack them. Karma!