Hi louisxp, welcome to the forum, and thank you on behalf of the animals at Gozo SPCA for becoming a premium member
penthouse - the top floor in a block of flats - they are generally smaller sized inside than lower floors, but with larger terraces - the problem with penthouses is that they tend to be hot in the summer and cold in the winter compared to lower floors, but with the upside of better views. They are also more likely to suffer from leaks. Being the penthouse doesn't necessarily mean you will have access to the roof, plus not all blocks of flats have lifts
maisonette - a place in a block, on one floor - an apartment really, but with your own entrance
terrace - a line of houses - can be detached or semi-detached
airspace - believe it or not, you can own a property, but not necessarily the airspace immediately above - someone else could build above you