Implement web Service in my Architecture

I'm really really new programing with an architecture, so I hope someone here can help me.

I have a solution on C# .Net with the next architecture:

• Data: Contains the UoW, Repositories, Entity Framework mapping and context.
• Design: It'll contain the WPF and Web projects.
• Domain: Contains entities that all the other projects will be using.
• Service: HERE IS MI PROBLEM!! I have no idea what to do here.

I need to implement a web service that will be consumed by WPF and WEB projects but I don´t know if it really goes here and how it needs to be (Web API or WCF).