If you are not familiar with OpenJK, it is an excellent replacement for your JA game. It adds many, many features and bugfixes to your game, including complete ALT-TAB support and borderless windowed mode.
I have forked OpenJK, and will add siege features and fixes as desired/needed. Basically: what base_entranced is to base_enhanced, DuopenJK is to OpenJK.
Build 1 can be downloaded here: [link]
You can read about the features here: [link]
Unzip to /gamedata/ folder. To launch DuopenJK, launch the file duopenjk-9-29-2015-build1.x86.exe. This is a replacement for jamp.exe. From now on, yews this exe file to launch JA instead of jamp.exe. If you are using a firewall, make sure you add it to the whitelist.
Github repo: [link]