if (!IsTileFlat(tile) || !IsTileFlat(tile2)) {
//		int z_upper_corner_n = TileHeight(tile);
//		int z_upper_corner_s = TileHeight(TileAddByDiagDir(tile, AxisToDiagDir(axis == AXIS_X ? AXIS_Y : AXIS_X)));
		int z_slope1;
		Slope slope1 = GetTileSlope(tile, &z_slope1);
		int z_middle_corner_n = TileHeight(tile2);
		int z_middle_corner_s = TileHeight(TileAddByDiagDir(tile2, AxisToDiagDir(axis == AXIS_X ? AXIS_Y : AXIS_X)));
		int z_slope2;
		Slope slope2 = GetTileSlope(tile2, &z_slope2);
//		int z_lower_corner_n = TileHeight(tile2 + (axis == AXIS_X ? TileDiffXY(1, 0) : TileDiffXY(0, 1)));
//		int z_lower_corner_s = TileHeight(TileAddByDiagDir(tile2 + (axis == AXIS_X ? TileDiffXY(1, 0) : TileDiffXY(0, 1)), AxisToDiagDir(axis == AXIS_X ? AXIS_Y : AXIS_X)));

		/* Forbid these combinations. */
		if (IsSteepSlope(slope1) || IsSteepSlope(slope2)) return_cmd_error(STR_ERROR_SITE_UNSUITABLE);
		if (slope1 == SLOPE_NS && slope2 == SLOPE_EW || slope1 == SLOPE_EW && slope2 == SLOPE_NS) return_cmd_error(STR_ERROR_SITE_UNSUITABLE);
		if (z_middle_corner_n != z_middle_corner_s && (IsSlopeWithThreeCornersRaised(slope1) && IsSlopeWithOneCornerRaised(slope2) || IsSlopeWithOneCornerRaised(slope1) && IsSlopeWithThreeCornersRaised(slope2))) return_cmd_error(STR_ERROR_SITE_UNSUITABLE);
		if ((slope1 == slope2 || slope1 == ComplementSlope(slope2)) && IsInclinedSlope(slope1)) return_cmd_error(STR_ERROR_SITE_UNSUITABLE);
		if (((IsSlopeWithOneCornerRaised(slope2) || IsSlopeWithThreeCornersRaised(slope2) || slope2 == SLOPE_EW || slope2 == SLOPE_NS) && IsInclinedSlope(slope1) && DiagDirToAxis(GetInclinedSlopeDirection(slope1)) == axis) || ((IsSlopeWithOneCornerRaised(slope1) || IsSlopeWithThreeCornersRaised(slope1) || slope1 == SLOPE_EW || slope1 == SLOPE_NS) && IsInclinedSlope(slope2) && DiagDirToAxis(GetInclinedSlopeDirection(slope2)) == axis)) return_cmd_error(STR_ERROR_SITE_UNSUITABLE);

//		int corners_tile1 = IsSlopeWithOneCornerRaised(slope1) ? 1 : IsSlopeWithThreeCornersRaised(slope1) ? 3 : 2;
//		int tee = TileHeight(tile);
//		int z_tile1_min = GetTileZ(tile);
//		int z_tile1_max = GetTileMaxZ(tile);
//		int z_tile2_min = GetTileZ(tile2);
//		int z_tile2_max = GetTileMaxZ(tile2);
//		bool z = Delta(z_tile_lower, z_tile) == 0;
		if (slope1 == SLOPE_FLAT || slope2 == SLOPE_FLAT) {
			/* Mark the tile as already cleared for the terraform command.
			 * Do this for all tiles (like trees), not only objects. */
			ClearedObjectArea *coa = FindClearedObject(slope1 == SLOPE_FLAT ? tile2 : tile);
			if (coa == NULL) {
				coa = _cleared_object_areas.Append();
				coa->first_tile = slope1 == SLOPE_FLAT ? tile2 : tile;
				coa->area = TileArea(tile, axis == AXIS_X ? 2 : 1, axis == AXIS_Y ? 2 : 1);
			}
			/* Hide the tile from the terraforming command */
			TileIndex tile_flat_before = coa->first_tile;
			coa->first_tile = INVALID_TILE;
			ret = DoCommand(slope1 == SLOPE_FLAT ? tile2 : tile, slope1 == SLOPE_FLAT ? z_slope1 == z_slope2 ? slope2: ComplementSlope(slope2) : z_slope1 == z_slope2 ? slope1 : ComplementSlope(slope1), z_slope1 == z_slope2 ? 0 : 1, flags | DC_NO_WATER, CMD_TERRAFORM_LAND);
			coa->first_tile = tile_flat_before;
			if (ret.Failed()) return_cmd_error(STR_ERROR_SITE_UNSUITABLE);
			cost.AddCost(ret);
		} else {
			if ((IsSlopeWithThreeCornersRaised(slope1) || IsSlopeWithOneCornerRaised(slope1)) && (IsSlopeWithThreeCornersRaised(slope2) || IsSlopeWithOneCornerRaised(slope2))) {
//				int z_middle_corner1 = TileHeight(tile2);
//				int z_middle_corner2 = TileHeight(TileAddByDiagDir(tile2, AxisToDiagDir(axis == AXIS_X ? AXIS_Y : AXIS_X)));
				
				/* Mark the tile as already cleared for the terraform command.
				 * Do this for all tiles (like trees), not only objects. */
				ClearedObjectArea *coa = FindClearedObject(tile);
				if (coa == NULL) {
					coa = _cleared_object_areas.Append();
					coa->first_tile = tile;
					coa->area = TileArea(tile, axis == AXIS_X ? 2 : 1, axis == AXIS_Y ? 2 : 1);
				}
				/* Hide the tile from the terraforming command */
				TileIndex tile_3corner1_before = coa->first_tile;
				coa->first_tile = INVALID_TILE;
				ret = DoCommand(tile, IsSlopeWithThreeCornersRaised(slope1) ? ComplementSlope(slope1) : slope1, IsSlopeWithThreeCornersRaised(slope1) ? 1 : 0, flags | DC_NO_WATER, CMD_TERRAFORM_LAND);
				coa->first_tile = tile_3corner1_before;
				if (ret.Failed()) return_cmd_error(STR_ERROR_SITE_UNSUITABLE);
				cost.AddCost(ret);

				if (z_middle_corner_n == z_middle_corner_s) {
					/* Mark the tile as already cleared for the terraform command.
					 * Do this for all tiles (like trees), not only objects. */
					ClearedObjectArea *coa = FindClearedObject(tile2);
					if (coa == NULL) {
						coa = _cleared_object_areas.Append();
						coa->first_tile = tile2;
						coa->area = TileArea(tile, axis == AXIS_X ? 2 : 1, axis == AXIS_Y ? 2 : 1);
					}
					/* Hide the tile from the terraforming command */
					TileIndex tile_3corner2_before = coa->first_tile;
					coa->first_tile = INVALID_TILE;
					ret = DoCommand(tile2, IsSlopeWithThreeCornersRaised(slope2) ? ComplementSlope(slope2) : slope2, IsSlopeWithThreeCornersRaised(slope2) ? 1 : 0, flags | DC_NO_WATER, CMD_TERRAFORM_LAND);
					coa->first_tile = tile_3corner2_before;
					if (ret.Failed()) return_cmd_error(STR_ERROR_SITE_UNSUITABLE);
					cost.AddCost(ret);
				}
			} else {
				if (IsInclinedSlope(slope1) && (IsSlopeWithOneCornerRaised(slope2) || IsSlopeWithThreeCornersRaised(slope2)) || IsInclinedSlope(slope2) && (IsSlopeWithOneCornerRaised(slope1) || IsSlopeWithThreeCornersRaised(slope1))) {
					if ((IsInclinedSlope(slope1) && DiagDirToAxis(GetInclinedSlopeDirection(slope1)) == OtherAxis(axis)) || (IsInclinedSlope(slope2) && DiagDirToAxis(GetInclinedSlopeDirection(slope2)) == OtherAxis(axis))) {
						if (IsInclinedSlope(slope1)) {
							/* Mark the tile as already cleared for the terraform command.
							 * Do this for all tiles (like trees), not only objects. */
							ClearedObjectArea *coa = FindClearedObject(tile);
							if (coa == NULL) {
								coa = _cleared_object_areas.Append();
								coa->first_tile = tile;
								coa->area = TileArea(tile, axis == AXIS_X ? 2 : 1, axis == AXIS_Y ? 2 : 1);
							}
							/* Hide the tile from the terraforming command */
							TileIndex tile_inclined1_before = coa->first_tile;
							coa->first_tile = INVALID_TILE;
							ret = DoCommand(tile, IsSlopeWithThreeCornersRaised(slope2) ? ComplementSlope(slope1) : slope1, IsSlopeWithThreeCornersRaised(slope2) ? 1 : 0, flags | DC_NO_WATER, CMD_TERRAFORM_LAND);
							coa->first_tile = tile_inclined1_before;
							if (ret.Failed()) return_cmd_error(STR_ERROR_SITE_UNSUITABLE);
							cost.AddCost(ret);
						} else {
							/* Mark the tile as already cleared for the terraform command.
							 * Do this for all tiles (like trees), not only objects. */
							ClearedObjectArea *coa = FindClearedObject(tile2);
							if (coa == NULL) {
								coa = _cleared_object_areas.Append();
								coa->first_tile = tile2;
								coa->area = TileArea(tile, axis == AXIS_X ? 2 : 1, axis == AXIS_Y ? 2 : 1);
							}
							/* Hide the tile from the terraforming command */
							TileIndex tile_inclined2_before = coa->first_tile;
							coa->first_tile = INVALID_TILE;
							ret = DoCommand(tile2, IsSlopeWithThreeCornersRaised(slope1) ? ComplementSlope(slope2) : slope2, IsSlopeWithThreeCornersRaised(slope1) ? 1 : 0, flags | DC_NO_WATER, CMD_TERRAFORM_LAND);
							coa->first_tile = tile_inclined2_before;
							if (ret.Failed()) return_cmd_error(STR_ERROR_SITE_UNSUITABLE);
							cost.AddCost(ret);
						}
					}
				} else {
					if ((slope1 == SLOPE_EW || slope1 == SLOPE_NS) && (IsSlopeWithOneCornerRaised(slope2) || IsSlopeWithThreeCornersRaised(slope2)) || (slope2 == SLOPE_NS || slope2 == SLOPE_EW) && (IsSlopeWithOneCornerRaised(slope1) || IsSlopeWithThreeCornersRaised(slope1))) {
						/* Mark the tile as already cleared for the terraform command.
						 * Do this for all tiles (like trees), not only objects. */
						ClearedObjectArea *coa = FindClearedObject((slope1 == SLOPE_EW || slope1 == SLOPE_NS) ? tile : tile2);
						if (coa == NULL) {
							coa = _cleared_object_areas.Append();
							coa->first_tile = (slope1 == SLOPE_EW || slope1 == SLOPE_NS) ? tile : tile2;
							coa->area = TileArea(tile, axis == AXIS_X ? 2 : 1, axis == AXIS_Y ? 2 : 1);
						}
						/* Hide the tile from the terraforming command */
						TileIndex tile_oppositecorners_before = coa->first_tile;
						coa->first_tile = INVALID_TILE;
						ret = DoCommand((slope1 == SLOPE_EW || slope1 == SLOPE_NS) ? tile : tile2, (slope1 == SLOPE_EW || slope1 == SLOPE_NS) ? IsSlopeWithOneCornerRaised(slope2) ? slope1 : ComplementSlope(slope1) : IsSlopeWithOneCornerRaised(slope1) ? slope2 : ComplementSlope(slope2), (slope1 == SLOPE_EW || slope1 == SLOPE_NS) ? IsSlopeWithOneCornerRaised(slope2) ? 0 : 1 : IsSlopeWithOneCornerRaised(slope1) ? 0 : 1, flags | DC_NO_WATER, CMD_TERRAFORM_LAND);
						coa->first_tile = tile_oppositecorners_before;
						if (ret.Failed()) return_cmd_error(STR_ERROR_SITE_UNSUITABLE);
						cost.AddCost(ret);
					} else {
						/* Shouldn't reach here. */
						assert(false);
					}
				}
			}
		}
	}