From The Heroes of Might and Magic III wiki

Adventure map objects that can't be passed until you've been to the correct Keymaster's Tent. There are eight colors of of Border Gates-- Red, Green, Light Blue, Dark Blue, Yellow/Brown, Purple, White and Black-- and you can't pass until you've been to a Keymaster's Tent of the same color. Unlike Border Guards, Border Gates do not disappear when someone wishes to pass through them.