Rampart is, essentially, a castle defense game where you go through three stages of gameplay. The pre-stage at the beginning of a game requires you to select a castle as your home castle, to which you will get walls built around it to own territory for the next stage. Then you skip to Stage 2 (as that was the stage 1 for the start of the game), building cannons. You gain 3 cannons to start and can only build them on your owned territory. Stage 3 spawns ships in the water whom you must destroy or last for 30 seconds, to which you will head to Stage 1, repairing holes in walls to re-gain territory and to gain new castles for construction bonuses and more cannons to build for Stage 2. The cycle repeats until you have lasted a fixed number of rounds or you capture all 5 castles.
Rampart on Wii will also support a two-player mode. In two player, the map is set so that there are two 'islands' where the players have their own sets of castles. The gameplay plays the same as single player, except they must destroy each others' walls and can destroy the cannons of their opponents (unlike SP, where cannons cannot be destroyed). A winner is declared when either a player does not have a castle after build period, or a player captures all castles on their side.