private boolean isFree(int x, int y) throws GameFieldException {
		assert checkPosition(x, y);
		if (units[y][x] != null) {
			return false;
		}
		switch (map[y][x]) {
		case MOUNTAIN:
		case WATER:
		case TREE:
			return false;
		case GRASS:
			return true;
		default:
			throw new GameFieldException(String.format("Unhandled type %s in function isFree.", map[y][x]));
		}
	}