How to reference a namespace from a specific assembly?

Let’s throw the specific answer up here in case someone comes along later.
From article here.

• Select one of the two assemblies under project references (in my case I selected Castle.Core). Hit F4 to bring up properties and enter alias CastleCore
• At the top of the problematic cs file put extern alias CastleCore;
• Reference your class with CastleCore::Castle.Core.Interceptors.IInterceptor. Or in my case I simply did:

using cci = CastleCore::Castle.Core.Interceptors;

and can now reference

cci.IInterceptor