Originally from a post by bugreports
here:"Another Portcullis Gate (southeast of the traps at K and L) cannot be activated or opened in any way. Therefore the chest at the end of that corridor cannot be accessed (except with use of Console commands such as tcl)." (Oblivion Remastered This issue has been addressed in Oblivion Remastered; the Portcullis Gate now contains an average-level lock that can be picked, along with the chest behind it.)
https://en.uesp.net/wiki/Oblivion:Fort_Pale_Pass
UOP already fixes one of the static portcullis. "Corrected a portcullis in the Mouth of the Serpent (Pale Pass) being a static, so it couldn't be opened and the chest beyond it was unreachable" (UOP changelog), but it does not seem to fix the other.