The Drag[en]gine is a fully customizable game engine and game development environment designed with modularity and extensibility in mind not requiring expensive licenses.
Portal system mesh as created in Blender. To look inside the ceiling of the top floor has been removed for this screenshot. Green are walls, red and blue portals connecting rooms. Rooms can be concave and the entire mesh is just a crude approximation of the visibility and is therefore simple and fast to create. No pre-processing of any sort required. Just export and plug it in the editor: done!
EDIT: A little clarification. This is just the mesh used to export the portal system. This is not the mesh used for rendering. The two are different. Models are exported to *.model while Portal Systems are exported to *.posys . The ISG HQ actually composes of 1 Portal System and 3 individual models per floor (left-wing, stairs, right-wing)