Eustachian tube opens into middle ear cavity at-
The tympanic end of the eustachian tube is bony and is situated in the anterior wall of middle ear. The pharyngeal end of the tube is slit like and is situated in the lateral wall of the nasopharynx, 1–1.25 cm behind the posterior end of inferior tubinate.Q

​Ref. Dhingra 6/e, p 5, 5/e, p 6; Scott Brown 7/e, Vol. III p 3114 Fig. 225.13
Anterior walls
Hypotympanum
Superior surface
Posterior wall