Linux support would be excellent. Write for for SDL:
Simple DirectMedia Layer is a cross-platform multimedia library designed to provide low level access to audio, keyboard, mouse, joystick, 3D hardware via OpenGL, and 2D video framebuffer. It is used by MPEG playback software, emulators, and many popular games, including the award winning Linux port of "Civilization: Call To Power."
SDL supports Linux, Windows, Windows CE, BeOS, MacOS, Mac OS X, FreeBSD, NetBSD, OpenBSD, BSD/OS, Solaris, IRIX, and QNX. The code contains support for AmigaOS, Dreamcast, Atari, AIX, OSF/Tru64, RISC OS, SymbianOS, and OS/2, but these are not officially supported.
SDL is written in C, but works with C++ natively, and has bindings to several other languages, including Ada, C#, Eiffel, Erlang, Euphoria, Guile, Haskell, Java, Lisp, Lua, ML, Objective C, Pascal, Perl, PHP, Pike, Pliant, Python, Ruby, and Smalltalk.
SDL is distributed under GNU LGPL version 2. This license allows you to use SDL freely in commercial programs as long as you link with the dynamic library.
If Civilization: CTP (and Heroes III, and Railroad Tycoon II...) can be written so as to run on Linux, GalCIV II should be doable. I'm not saying that it's easier or simple just doable. The bonus of writting against SDL is code once, compile three times and you have instant ports for Linux and MAC, in addition to your Windows version. I realize that Stardock works closely with Microsoft, but that shouldn't be a reason not to allow us other OS users to enjoy the game, should it?