There is nothing similar to rails console in sinatra. You can go to irb and do a :
require './myapp.rb'
to start experimenting with your Sinatra app.
You can create a console.rb and put the following code:
require './game.rb'
require 'irb'
ARGV.clear
IRB.start
You now have a simple Sinatra console, you can run it as ruby console.rb.
Reference:
Sinatra Console
