One of the best part of this game is that you don't have to play every level, as the game will unlock 3 levels after the one you have just completed. For example if you finish level 5, you can play levels 6, 7, and 8. If 7 is too difficult, you can play level 8 and unlock levels 9, 10, and 11. You do have to play the final level in order to see the ending though.
The challenging part is the various weapons/items on the map would have a different effect each time you loaded a level. So you could play level 5 one time and the Electrolaser does Medium Damage. The next time you play level 5 the Electrolaser will do Multiply effect (hit one alien, it turns into two aliens). The time after that the Electrolaser has the Shrink effect (Alien -> Grub -> Cocoon -> dead). So even the same mission will require different tactics.
I would like this game on GoG as the CD is starting to fail and the game no longer installs on Win10.