The most Tower Defense games use Top Down-view. Not Sanctum! Sanctum is a mix between a Tower Defense-game and a First Person Shooter. This concept makes the game interesting to play with lots of fun!
The enemies are different kinds of aliens, who all got different powers and capabilities. There are a lot of species; small, fast bugs, who operate in high numbers, or flying powerful raptors, anything's possible! All the aliens have weakspots, which, if you shoot them, does more damage to the alien. Every aliens' goal is to destroy the core, which you have to defend. The aliens arrive in waves; On the easy maps, there are just a few aliens but later on, hundreds of small creatures are spawning! To help you a little bit, before the game starts, you gain a few resources; a small amount of bases and some currency to build towers with. When you complete a wave, as a reward, some more resources are added, so you can improve your towers to upgrade them or to build some more bases.
There are four characters, which are all customizable with other weapons and perks, but also the placeable towers are shown. Furthermore, you gain EXP which maintains in higher levels. Every level, you obtain new perks/weapons/towers. Perks are, for example, extra damage, more core health, and many more. Weapons include shotguns, SMG's, electricity guns etc., and there's a wide range of towers; Mines, cannons, drones, lasers, there's even a tower that increases the damage of nearby towers! All these options makes it fun to tweak your character and to try every perk/weapon/tower.
There are lots of maps to play. Every map feels different, and has a different route from the spawn of the aliens, to the core. Every map is original and doesn't feel the same at all. The trick is to make the route, from the spawn of the aliens to the core, as long as possible. That means you've got to design a maze, but you haven't got unlimited towers; only a few. So, you have to really think while the game hasn't even begun yet!
This game is even more fun in co-op! People can join your singleplayer campaign when they want (so you don't have to start over). Just set, while or before you're playing, your level to 'Public', so everyone can join! There also options if you want to play at your own, or if you only want to invite friends into the match, and not random strangers.
Also, there's some DLC available. I bought the season pass with all the DLC, and the skins pack, so I'll taslk about the DLC as one part, and not every part by itself. The DLC is really worth it! It contains of four new maps, together with an assload of weapons, and a ♥♥♥♥load of perks, and lots of new towers, together with a whole new personage to play with, called TSYGAN. The new weapons are very different than the ones you've been playing with in the main game, and they're a nice addition. This also counts for the towers and the perks. However, there are so many perks now that it could be too much to choose from. If you bought the DLC, some more achievements that you couldn't get before, are now available. This makes Sanctum 2 even more addictive because you really want to get all the achievements. (And that takes some time!)
Actually I don't have any big flaws, except that the game can get boring over time, and that a few levels (escpecially with some Feast of Strength; perks for the aliens who attack you) are really difficult if playing alone. Also, if you haven't bought the DLC, some achievements can't be completed; You have to buy the DLC to be able to earn all of them. However, the DLC is quite cheap, especially if it's on sale, and it's definitely worth it!