To build a server-side Swift application that plays Pokémon using text messages, you'll need to install wine, Vapor, and a Windows Visual Boy Advance emulator with Lua scripting capabilities. You'll write Lua code that reads from a file called "button.txt" on every frame of the game and writes to it based on button presses received via text messages. Then, you'll create a new Vapor project and add a route to receive SMS messages to your Twilio number for button presses. With ngrok, you can make your app publicly accessible and configure your phone number to start receiving text messages from friends who want to play Pokémon with you.