It's probably a custom format, made by the developers of Archlord. Maybe you could post/link to a file, so people can take a look at it.
The most simplest thing is that it could be a normal archive, with some modifications. Project Entropia (now called Entropia Universe) had something similar shortly after release. They used simple ZIP files to store the files, with one little modification: The Magicnumbers for ZIP format was changed, so that a normal ZIP unpacker couldnt extract them (Original ZIP format had a "Pk" letters, at begining of each new file info. Mind Ark changed it to "Ak" so WinZIP etc. couldnt open it. But if you used a hexeditor and changed all "Ak" to "Pk" there was no problem opening that file in WinZIP and extracting its content).
Later they used BNT/BNT2 Formats, that all files was written in a sequence with no delimiter chars, but at the end of the file, there was a Table of Content, which had filename, start offset and size stored and with that it was possible to write a programm to extract the BNT files.
Warhammer Online Client for example, uses myp files. The table of content there is split in blocks of ~100, containing packed, unpacked size, start offset as well as offset of the next "100"-table of content chunk info. The filename itself, was stored in at an extra header (which starts at the offset from that toc). The content of the file itself, was packed using gzip libraries/algorythm.
So its hard, to tell you how the Archlord files are built up, w/o taking a look at it. You can also try to open it in a hexeditor and look if you get some clues.