Randomizing Enemy locations and numbers would add a bit of freshness. Knowing what enemies are behind each door and what they're armed with does kill the suspense a bit.
Beyond the portal things seem fine to me, the enemies are able to roam the hallways enough that you can't predict where they...
Explore, fight, explore, fight, run away, run away, repeat.
"Hey, you know what would improve this game play?"
Explore, fight, sit in chair............, run away, run away, sit in chair............, repeat.