Oh, I hear you. The initial plan was to put more effort into more types of attacks, and various advantages/disadvantages. But time pressure caused us to shift tactics on what to implement. Madam Materia did great on the sprites and story, so my main priority became to make sure all those made it into the game.
The "def" part is working as intended, the displaying of terrain info just isn't as clear. Being on terrain just gives a bonus to defense, but when planning an attack, it doesn't update this bottom line to show the (adjusted) defense of the target. And flying units just never get this bonus, but if you attack from a flying unit to something in mountains, the unit in the mountains still gets a defense bonus.
If you grab the windows version, you actually can discover that a lot of this is data driven and can be adjusted from text files. There is actually also an editor, but I disabled that for the jam release. You can find that here: https://daid.eu/8bit/TrialsOfOlympus.html if you want to create your own custom challenge ;-)