Land mines

[gokusiya]

i suggest
landmines
they could be hidden and seen by the player who placed them.other players could use specialized equipment to see them
and there could be various types of mines anti tank for example.

[Giani]

There exist mines almost as the same way you said, in the NTW mod.

[Delphinio]

http://ntw.net.ms or http://www.moddb.com/games/ntw