Hello, Yormeister ...
Yes, the 4-way connectors work fine with the old Steck-System castle.
However, since they were intended to be used with Steck-System wooden buildings, they are not castle gray.
AND, they are not all the same! The one that I think that you want (for the Steck-System Castle) is 30 05 3070 (from Farm).
All the best,
Richard