Shadowgate Classic refers to several released games by Infinite Ventures in 1999 (a group made of previous ICOM staff). The main release was an updated remake made for IBM PC (Windows '95/'98/2000/Me) and portable systems (Windows CE P/PC and Pocket PC). The latter version included the extra rooms (Armory) of the original versions (B/W Mac or color Atari ST) with new 256 color artwork for the backgrounds and inventory items. The inventory is based on the original Mac and Atari menu.
Shadowgate Classic is also the name of a modified port of the NES version of the game for Gameboy color. There are some changes to the graphics such as character sprites, and improving the ending cutscene/background. But some animations were removed in the process.
Another version Shadowgate Classic Mobile was made for cell phones in 2005 was a completely new remake with original graphics. But apparently had a few puzzle changes as well, including the Ice Elemental added to the former Pedestal Room.