Indeed—the CD in HOMM3 is a very inconvenient unit—expensive and slow (in the sense that by the time you build everything necessary in the Castle, the Barbarian monsters will have been growing for a month at 3 per week)

Building layout
1. Stronghold
2. Castle
3. Wolves
4. Ruhs
5. Monsters!
Another 2 days for accumulation and to build something else to your liking
(Orcs+Ogres, for example)
And if the wolves were already in the castle, you could even build Cyclops.
Naturally, on a RICH map with sufficient resources.
But for the CD, ALL creatures before it are needed + 2 levels of the Mages' Guild (red)—even with abundant resources—that's only the second week!

Furthermore, Barbarians require wood+ore+crystals, while the Dungeon requires all resources!