In dungeon keeper rooms have efficiency, the more efficient the room the more it can hold (gold, traps, creatures), the faster it delivers(training, traps, etc) and the workshop even needs to reach max efficiency to build to most powerful traps and doors. Make your room large, reinforce the walls and build doors to improve efficiency, shown by the 'grey bar' of the room flag and the size of the room flames surrounding it.
The reason for this is simple, if room efficiency did not exist you would be best of simply don't building any walls, putting the rooms right next to each other. This would be the most efficient use of space, would allow creatures to walk fastest from one room to the next and give you maximum flexibility to change rooms around.
Now there's a big mistake, that has been around since release of the original game: Guardposts and Bridges have the exact same effect as doors and fortified walls.
Making this the best dungeon you can make:
Notice all the rooms are at maximum efficiency. You can even assign creatures to the workshop to patrol your dungeon.
And if you don't like open plan, you should also replace all your internal doors with guardposts:
Not only will you get guardposts before you can make doors and does this save you the trouble of building the workshop, a guardpost is also just 50 gold where you can sell doors for at least 250 gold a piece. Beyond that, training room doors take damage, guardposts do not.
Now confess, who's been using walls like an idiot for almost 20 years?
(By the way, I put in a change request, and for the upcoming version of KeeperFX guardposts/bridges can no longer be abused like this.)