In a few games, you will want to let players alter the terrain. This needs to be done judiciously, since a cell of terrain generally represents a vast area, and the simulated time in Xconq is generally too short for major terraforming operations. However, building bridges and digging moats can be reasonable additions to a game.
Since actions are always completed quickly, and there is no concept of "partly modified terrain", you will probably have to come up with a trick to make terrain modification be slow. One way is make the acp (or material?) cost very high. Another way is to make the alteration happen by removing a material, such as clearcutting a forest, then letting the action make the actual change to clear terrain.