Fires happen, we can't really expect to cause any more mayhem than your average pyromaniac, and cities survive those, so I don't really see as it will get the fort to lower its defences.
Some assumptions:
We cannot beat anyone who is likely to possess the amulet in a direct confrontation.
We cannot go over the fortress walls without assistance or training.
We lack the time for training.
The front gate cannot be safely bypassed if relying upon visual illusions.
A lone agent cannot personally cause sufficient mayhem to appreciably tax the city's resources.
We need that amulet.
The adventurers are not going to die horribly, but leave all their look lying around for us to safely collect, no matter how much we follow them...
Well, options that I can see:
Gain help from others. I don't really see a good way to do this. We could try impersonations, but we don't know the political environment well enough, we have no fortune to spend on hirelings, and have no friends. But a well-placed illusion might trick someone into doing something beneficial to us for their own reasons... Perhaps spread a rumour that a powerful faction wants the amulet, it might get it stolen, which would destabilise its security and likely keep it from travelling or being dismantled, but would likely increase its security too...
Find a alternate access to the fortress, see if there is a less guarded entrance, or an escape passage, but that is very unlikely. Tie a mess of birds together and use them to gain some altitude from which to glide into the fortress. Hot air balloon?
Stake out the fortress and practise our climbing, keep following the amulet until we are strong enough to take it...
Incite a riot against the fortress...
Predict the path of the adventurers, dig a hole along that path, place magma spiders poisoned spikes flammable oil in the bottom of the hole, wait...
P.S.
A single out-of-control fire is not going to burn down a city, people survived the middle ages...