Now it's time to replace those weird trees inside of the building structure with a cave entrance.

Select Combo #128 in the combo list. If you hover the mouse cursor over the combo in the list, it will give you some brief information.

Make sure the combo you select is showing as Cave (Walk Down) [A]. The [A] is especially important. Now fill the combo in over the two trees of the structure.

If you hover the mouse cursor over the cave entrance you just created, it should show a tooltip that confirms this as combo 128 with the given CSet and other information. The Pos is the position of the combo on the screen out of the 176 combo positions available on the screen (0-175). For the moment, we are going to ignore Flags in that tooltip, though both values should be 0.

Since this is going to be an item cave like Zelda 1, we need to set it as such. Go to Screen→Room Type.

Select the Room Type Special Item.