OK, I got this down now....
The tower objects, such as twr0 and twrv0, are actually 2 objects in 1 package. The O'Club interior room is part of the tower object, down there inside it, which you can see if you zoom in on the tower in the OE. However, despite both being added in 1 go as a single object in OE, the upstairs tower room and the downstairs interior O'Club room are treated as 2 separate entities by the game.
To make the O'Club function as intended, you must leave the Overide (sic) Type field in OE set to "all/default" when you add the tower object to your field. This lets the default values for both the upstairs tower and the downstairs O'Club get set as HTC intended, 1 as the tower and the other as the O'Club, so that both function correctly once you build the map and try it in-game. If you set the Overide (sic) Type field to "tower", then it sets BOTH the upstairs tower and the downstairs O'Club to towers, so you can't get inside the O'Club.