James Closs, Developer in Bath, United Kingdom
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James Closs

Verified Expert  in Engineering

Mobile App and Game Developer

Bath, United Kingdom
Toptal Member Since
March 1, 2022

James is an award-winning digital media professional with 20+ years of experience specializing in cross-platform mobile app and game development and design using C#, Xamarin, and related technologies. James has developed apps on everything from the first Java-enabled mobile phones to contemporary game consoles and consulted at a senior level on major projects for high-profile clients, including Disney, Nokia, Cartoon Network, Open City, and Electronic Arts.



Preferred Environment

MacOS, Windows, Visual Studio .NET, Xamarin, Monogame, Xamarin.Forms

The most amazing...

...app development project I've completed was for the high-profile Open House London event, which attracted over 25,000 users during one weekend.

Work Experience

Creative and Technical Director

2000 - PRESENT
BitBull Ltd
  • Developed cross-platform mobile applications for clients using Xamarin.Forms, including a Tamagotchi-style app to help children manage and track their screen time and a fitness tracker for a sports equipment manufacturer.
  • Designed and developed Jetboard Joust, a retro-style arcade game, and ported it to all major desktop platforms and game consoles using C#, MonoGame, and associated technologies.
  • Built 30+ mobile games for Java and Android phones, which were served on all major international operator portals, regularly featured in the ELSPA charts, and won several Pocket Gamer awards, including a Game of the Year nomination.
  • Developed a Java-based middleware to enable J2ME/Java ME applications to run seamlessly in Java AWT desktop and Android environments with no code changes.
  • Created an ANT-based build tool to enable 1,500+ mobile handsets of wildly varying specifications to be targeted from a single codebase and XML configuration file at the press of a button.
  • Developed a Java framework for mobile and browser-based games. Aspects of the framework were licensed to Digital Bridges and Nokia for a six-figure sum.
  • Built a browser-based dating application that included a multiuser live chat with virtual rooms and fully customizable avatars.
Technologies: Java, C#.NET, J2ME, Monogame, Games, Game Design, Game Development, Mobile Game Design, Console Games

App Developer

2017 - 2020
Open City Architecture (Freelance)
  • Developed the app to accompany the annual, high-profile Open House London event that attracted over 25,000 users in one mission-critical weekend.
  • Collaborated with the back-end developer to specify the REST API that would give users access to comprehensive information on the approximately 1,000 participating buildings in a robust and efficient manner.
  • Worked closely with the art director to ensure the integrity of the UI design across iOS and Android while maintaining a 98% shared codebase.
  • Integrated Xamarin.Forms.GoogleMaps, an open-source, cross-platform solution for using Google Maps with Xamarin.Forms.
Technologies: Xamarin.Forms, JSON

Creative Consultant

2002 - 2003
Fjord (Freelance)
  • Worked on high-level conceptual design and storyboarding for a major new communities service for Orange (Hutchinson Telecom).
  • Developed concepts for a series of augmented reality games for Nokia.
  • Developed concepts and designed the UX for several mini-games for Nokia to promote its sponsorship of Disney's Tron Legacy movie.
Technologies: Creative Concept Development, UX Design

Creative Director

1995 - 2000
  • Developed the creative concepts for numerous pitches, winning contracts from high-profile clients, such as Disney, Pepsi, Barclays, WPP, JWT.
  • Developed the concept and oversaw the development of a browser-based, massively-multiplayer online game for Cartoon Network, probably the first of its kind using early web technologies.
  • Concepted and oversaw the creative development of the first online eCommerce presence for Boots, the leading UK pharmacy-led health and beauty retailer.
  • Developed the concept and oversaw the creative development of the first significant online presence for BP using early personalization technologies.
Technologies: Art Direction, Creative Direction, UX Design, UI Design, Concepting, Web Technologies, Multimedia Design, Game Design, Game Development, Java, JavaScript

Open House London App

Open House London is an annual event in which 800+ buildings of architectural significance are opened to the public for one weekend. The app enabled users to define comprehensive search criteria by which listings could be filtered, maintain a list of favorites, and plan daily itineraries. It also included full mapping functionality and the ability to search for buildings of interest nearby.

I was the sole developer of the app that accompanied the event and maintained it from 2017 to 2020, when the event was scaled back due to the COVID-19 pandemic. The app had regularly attracted 25,000+ unique users across the event weekend, and because of the time-sensitive nature of the project, it was absolutely critical that it ran without bugs.

I built the app in Xamarin.Forms with the vast majority of the codebase consistent across iOS and Android. I worked closely with the UI designer to maintain the integrity of the design across platforms and with the back-end developer to specify the REST API used to retrieve information on the participating buildings. Finding ways to asynchronously load large amounts of data without compromising the user experience was a key challenge.

Goozby | Screen Tracker App for Kids

A Xamarin.Forms-based app prototype that featured Goozby, a Tamagotchi-style character, to encourage children to spend time away from their phones. Points were awarded for the time when the phone was screen locked, which enabled the character to grow.

The app required a native Xamarin component on each platform to maintain a background process that tracked the locking and unlocking of the user's screen. Users could challenge themselves not to exceed a certain amount of screen time per day and would receive feedback from their Goozby based on how they were performing.

Mobile Game Engine and Games

A framework I developed for the production of mobile games, initially for Java feature phones but later ported to C# and Xamarin for iOS and Android. I created 30+ games using this framework, which collectively sold over half a million copies. Aspects of the framework were licensed to Digital Bridges (now I-play), Electronic Arts, and Nokia.

The framework included an emulation layer that enabled J2ME games to run in a Java AWT environment for desktop and browser and on Android phones with no code changes. It also included an ANT-based build tool that would target 1,000+ handsets with wildly different specifications from a single XML configuration file.
1992 - 1994

Postgraduate Diploma in Graphic Design with Computing

Central Saint Martins School of Art - London, England, UK

1989 - 1992

Bachelor's Degree in Graphic Design

Central Saint Martins School of Art - London, England, UK


Visual Studio .NET


Monogame, .NET


Java, C#.NET, C#, XAML, JavaScript


MacOS, J2ME, Xamarin, Windows


UX Design, Mobile Game Design, App Development, UI Design




Xamarin.Forms, Interactive Media Design, Art Direction, Creative Direction, Concepting, Game Design, Game Development, Creative Concept Development, Games, API Integration, Graphic Design, Typography, Computer Graphics, Multimedia, Console Games, Multimedia Design, Web Technologies, Mobile Game Development

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