Even though there is only one plane of existence, I hope certain places, like the Calebethon for instance, are smaller mini-maps with at least one level below or above the surface of "the outside world".
For instance, a grand castle on the map takes up one square ... and entering the castle, instead of merely be a one-time tactical battle, would be a serparate map, at a different scale, representing the Trek through the Castle Halls. With several potential tactical battles at least.
Same could be done for caves and dungeons. Usually a different scale than the overland Strategy Map, with various quest items and tactical battles within. (A quest item could also result in a tactical battle).