It's been a while, so pardon me if anything I say was already said (even by me).

There used to be passwords to get to secret rooms in different buildings on the map.
Most memorable two were:
"Fear the tree" - Zanza guild house
"Rudora" - Skyld's treehouse (probably cause Rudora was his server..or at least one he worked on)

There was also one in Jezzle's House (which was located in Swamptown). Maybe one in the SOS gh or SNC gh. Anyway, the SOS building (near Jezzle's) had a secret room i think. Might've been for (The Society of Shadows) members only...similar to the secret room only (Zanza) family members could get into.

uhhhh the room on the left in the castle lobby was a secret (as in it wasn't marked that there was a room there). The hallway you need to bomb to get into in the castle wasn't marked very clearly either. One of the rocks in the building near Cinobi Park could be blown up to get to a level with a gralat chest. There wasn't a mouse hole near the fireplace in the Mad Hatter. I think I had a few others in mind, but i can't remember.