Creating a Custom Interface

How can I create a custom interface like God Wars II has done with your client?

They basically made a lot of plugins using Miniwindows. See the Miniwindows part of this forum. There are quite a few examples there (like chat plugins, health bars, etc.). The mapper plugin is another example of something that is moderately easy to add on. To make the mapper work properly without a lot of false room reports you need support from the server (ie. to know the current room number).