Add basic lighting, like torches on the walls, or on top of sticks in the outdoors etc, and also sun and moon lighting that changes intensity, perhaps color too, and angle of insidence as the time goes by.
Lights other than the sun and the moon would have a limited range, getting dimmer quickly past the range. The sun and the moon would hit all of the open areas and might enter thru windows and doors depending which way they face.
Light would get blocked by stuff, rock, walls etc, creating shadow areas (shouldn't be hard to calculate what areas would be lit, it's just a matter of mapping where the light source could reach in a straight line in all directions without hitting any light blocking stuff), as for the sun and the moon, for terrain, it would just be a matter of checking the angle of the terrain (so if it's facing away from the sunrise it's dark) and if possible also project shadows by running a a line from each tile using the angle of the sun or the moon in the sky and see if it can reach beyond the the highest level of the map without hitting more terrain nor constructions.
Tiles next to lit tiles would get a small bit lit as well for indirect illumination and smooth penumbra areas fallout, this would also result in lit tiles being a bit brighter if they have lit neighbours helping with making it look more realistic as well.
Areas that are too dark wouldn't be included in the path calculations unless the dwarf of creature has it in direct line of sight, then instead the path algorithm will check if the agent can see a lit area on the other side of the dark area that is closer to the the intended destination than the current position. Different species would have different thresholds for what is too dark, species that can see in the dark or that "see" by means other than light (echolocation, remote vision etc) would have the threshold all the way down, so even the darkest tile would still be visible for them. Perhaps show the lightness of the map as relative to the distance weighted average of the agents near by (so you can see how they see it), but if there is no one around for a certain distance, then revert to standard lighting level.
Doors and windows when open will let some light, sun and moon as well as artificial and bounced (indirect) light, but for the sun and the moon, it wouldn't let any direct light in when they are exactly on top (noon for the sun) since the light would be going straight down, that is unless you also model different longitudes (i always mix the two, longitude is the distance from the equator, right?), then doors and windows facing the north or the south would let a bit of light thru.
If possible also have the materials of the floor/ground and any neighboring walls as well as other objects in the tile influence how much the light will bounce from that tile, things that are darker bounce less light, things that are brighter bounce more, things that are reflective will bounce even more light.
Also if possible implement things like mirrors, light ducts etc to allow redirection of light; like for example using a mirror to bounce skylight into a duct of reflective material inside the mountain to bring natural light to deep rooms etc.
Perhaps offer the option of having adaptive pseudo HDR for the Look mode (k), where lighting level of everything in the screen gets increased or decreased by the amount required to make the tile where the cursor is be lighted in a regular brightness (not too dark, nor too bright, just enough to show it's colors unchanged)m , tiles too bright would get closer and closer to being white, and darker tiles would go black.
Another interesting addition, but that would use up a bit more processing power would be to have magma, fire and portable torches and perhaps some other moving/changing light sources (a crystal that emits light when under a certain temperature range, perhaps the horn of the Unicorn would emit light while the unicorn is alive (so people can't use it if they kill the unicorn or try to saw it off a live one), lightning during storms etc).
Perhaps for a later moment, improve the light model to work in RGB, so colored light sources and colored light reflected from objects would be possible. So when light from two or more light sources combine you add them together, and then the resulting color is multiplied by the color of the object/surface (with each color component expressed as a value between 0.0 and 1.0) to get the color the object/surface will look like, and also the hue of the light bouncing off it to neighboring tiles.