The game is played with a board and pieces. The object of the game is to capture ('kill') all your opponent's pieces and/or 'escape' your own, removing them from the board. Unless a player resigns, all the pieces of one side must be removed from play before the game is won.
The game board is made up of small squares, usually of alternating colour (although this is only to provide visual contrast: there are boards made of unpainted natural wood, too). Boards can be 8x8 or 10x10 squares.
Each side begins with the same number of pieces, in the same (symmetrical) positions. All the pieces of each player are identical, but there is no standard appearance. The pieces in some sets are little blobs or cubes or tiles. In others they are miniature humanoid figures, as intricate as you like.
Gameplay revolves around assigning rôles to your pieces, all of which start off as harmless pedestrians ('walkers'). The abilities of the various rôles include altering the terrain by building and destroying mounds or digging and refilling pits, as well as abilities that affect movement or fighting. Abilities can be lost or expire after a certain number of turns, and obviously, when a piece is killed its abilities are lost.
(The author isn't clear on this, but here's how de thinks it works.)
Embankments and ditches are represented on the board using different accessories depending on the game set; some come with miniature hills that can be placed on a square, some have reversible squares and some modern ones even use magical representations.
Similarly, a piece with an active rôle can be altered with a little hat, ring, differently-coloured base etc. Such optional accessories - by no means essential to gameplay and adding complication as well as increasing the risk of losing bits - seem to be a recent development, considering that oniva has been played for many hundreds of years.
Many players prefer the traditional way, keeping track of rôles by memory alone; this also adds the possibility that the opponent may forget your pieces' abilities.