In a lot of ways Sanctum 2 is a perfect sequel. It does everything that worked in the first game - the basic setup, the solid mix of mobs and towers, and then expands on it in nearly all directions. So whilst you have a build phase and a spawn phase, resources are generally tighter, meaning you need to make more choices about where you put things. Further to that, some of the build phases are timed, meaning you have to make those decisions faster. The levels are much, much more fiendishly designed, and speaking as someone who played the whole game in 3-player co-op, occasionally pretty taxing on the brain just to understand the flows through them.
The aliens too have been given an upgrade. Many of them come with new abilities, and some families - the Floaters, Heavies and Walkers - all have "Boss" variants what can take a lot of punishment to take down, and in some cases even destroy towers on their rampage through your territory. It makes for frantic spawn phases, and an additional layer of complexity to your planning. The last big change is that players now come in one of four specialised classes, which can be further tweaked by selecting "perks" (such as increased damage, or making the Core regenerate) and secondary weapons, not all of which seem to be a lot of use.
It is however a lot of fun. Where the first game seemed to run out of steam slightly, the sequel powers on through to the end, changing up not just the look of levels but also the sorts of tactics you need to defeat them, keeping you on your toes and staying interesting to the final, impressively tough, mission. With a bunch of DLC maps coming out, I'm sure to playing this more in the future, too.