You've asked a large question, so I'll break it down into steps. For brevity, I'll assume you're familiar with Unreal Engine's interface and basic concepts like Blueprints and Actor components.

Before moving on to the steps, I want to clarify that Unreal Engine can automatically handle much of the complexity of multiplayer networking for you, such as replicating Actor state and RPCs (Remote Procedure Calls), which are a fundamental concept in Unreal networking.

Enable Online Subsystem:

First, you need to enable the Online Subsystem of Unreal Engine. To do this, open your game's .ini configuration files and add these lines:

csharp
Copy code
[OnlineSubsystem]
DefaultPlatformService=Null
This sets up a basic, local online subsystem for testing purposes. In a production game, you might replace this with a different online subsystem, like Steam or Epic Online Services.

Make your Game Replicate:

Next, you need to set your game's Actors to replicate. This means their state (position, rotation, etc.) will be automatically synchronized between the server and clients. To do this, open each Actor in the editor, and in the Details panel, check the box that says "Replicates".

For your 2D top-down game, you likely want your player characters and any interactive objects (like power-ups or obstacles) to replicate.

Create RPCs for Player Input:

Player input is usually handled on the client side, but in an online game, this input needs to be sent to the server to affect the game world. This is done through RPCs.

To create an RPC in Blueprints, create a Custom Event, and in the Details panel, set its Replicates property to "Run on Server". Then, wherever you handle player input (usually in the PlayerController class), call this event.

Set up Game Mode and Player Controller:

The Game Mode is the class that defines the rules of your game. This includes which PlayerController class to use, which GameState and PlayerState classes to use, and the default pawn class for players.

The PlayerController is a class that receives input from a player's machine and can issue commands to the player's Pawn (the character they're controlling).

You might need to create your own subclasses of these classes to implement your game's specific rules and behaviors.

Build the Dedicated Server:

Once your game is set up to support multiplayer, you need to build the dedicated server.

First, close the Unreal Editor.
Open the Unreal Frontend tool (located in the same folder as the Unreal Editor executable).
Go to the "Profile" tab and create a new profile.
In this new profile, set the command to "[YourProjectName]Server". Set the map to your starting map and add the argument "-log" to the command line.
Choose your project under "Project" and set the build configuration to "Development Editor".
Under "Cook", select "By the book", add your maps and set the "Build" and "Cook" checkboxes.
Then, click on "Launch" at the bottom of the window. This will build your game as a dedicated server and run it.
Connect Clients to the Server:

Finally, you need to connect your clients to the server. Open two instances of your game, and use the "Execute Console Command" node in Blueprints to run the command "open 127.0.0.1".

That's the basic process of creating a dedicated server and connecting clients to it. Unreal Engine handles a lot of the networking complexity for you, but it's still a complex task that requires careful design and lots of testing. Remember to test your game frequently as you develop it, to make sure the multiplayer components are working as expected.