I've know Ali, who runs Iliada, for some years now and have a lot of his minis and terrain in my collection, so I thought it would be fun to depart from the regular schedule of elements and, instead, to look at some terrain
Image (c) Iliada Game Studio
Buildings in Horizon Wars: Midnight Dark can be basic or advanced terrain. If you are sticking to basic terrain, just make buildings solid and even inaccessible. They are nothing more than a blocker on the tabletop that obscures line or sight. But they become much more interesting when you used the advanced rules, because then infantry type elements can enter them.
First up, entering a building in itself is interesting, because the element gains full cover and, if they dig in, get even harder to shift. They can also draw a line of sight from any point on the building not just from their base (this represents an element spreading out in an occupied building to get the best vantage points). But things can get more interesting if you decide that a building is booby-trapped (ergo, dangerous) or perhaps damaged by shelling (making it treacherous).
Buildings like Iliada's Politburo HQ also become interesting because they are large. This means you generally don't want to treat them as one single building. Ali's design is especially useful in this respect because it comes in three clear sections that you can build as a single contiguous piece - as he has done in the image above - or as three separate buildings. If they are contiguous then elements of different sides can occupy one or more sections. But as the sections are touching that means the elments are engaged and must fight. So you get a vicious firefight going on inside the building as the sides fight to control them.
If you want to keep the theme of this being the former (or even current!) HQ of a political paramilitary force, then you will probably want it to be dangerous terrain. But avoid making it treacherous if the building needs to be an objective, because treacherous terrain makes elements exhausted and exhausted elements can't hold objectives!
As a side note, Iliada Game Studios is developing more 6mm miniatures, moving into elements that will be printed resin, so do check them out in the future for an expanding 6mm range.