First and Second secret room can be accessed through the corridor below the dark whirlpool on the left and right.
Third and fourth can be accessed through the corridor below the left and right Summoning pillar.
Enemy Reinforcements
The two mana pillars on the sides spawn Werewolves on turn 3
The upper two monster portals spawn Martels on turns 5, 10, 15, (not 20 or later)
The lower two monster portals spawn Werewolves on turns 5 and 10, 15, (not 20 or later)
Gates
The gate at V-163 can only be opened by defeating Oktavia.
The gate at T-171 requires the Bronze Key to open, which on a First Playthrough can only be obtained in later stages, making 100% occupation impossible on the first try.
When revisiting there is a small event if you go to the big room where Bridgette was (note about Bridgette). I still don't know what it might be related to, atwiki says nothing about it.