The unique Ateşgah is an 18th-century fire temple whose centrepiece is a flaming hearth. Arching above is a four-pillared stone dome with side flues which also spit dragon breath…but only on special occasions, notably the four Tuesdays leading up to Novruz (21 March). The fire-altar sits in a roughly triangular courtyard surrounded by the simple stone cells of former devotees. Well over a dozen cells now host a well-explained museum. Allow around an hour to see it all.
Although the site was originally a place of worship for Zoroastrians, the fortified complex you see today was built by 18th-century Indian Shiva devotees.
To get here, take bus 184 from Baku's Ulduz metro station to the north concourse of Suraxanı train station. To find the fire temple, cross the rail tracks through that station, turn left, go through a grandiose stone gateway then walk another three minutes passing a baronial hall–style cafe-restaurant.
If/when the mooted airport-railway service starts, a better option than the bus would be to take a train from direct from Baku to Suraxanı station.
Note that after 6pm you can't even enter the outer compound, so you won't be able to see anything outside of visiting hours.
