All things considered, Serious Sam: The Second Encounter is one of the most well-developed games of all time.
If you're familiar with Serious Sam games, then you know that the player will engage in spectacular combat sequences (often in a big, open environments), using powerful weapons to defeat hundreds of enemies that spawn from nowhere, and all of them almost at the same time. Sam Stone is usually surrounded by a lot of monsters, such as skeletons, bulls, beheaded kamikazes, headless soldiers... and all of them mindlessly attack the hero. So, basically, all you have to do is to dodge and shoot. But that won't grant you victory, as you need to use some strategy: for all the enemies you have a best response, both when attacking them or defending yourself. Not to mention that the levels are filled with obstacles, which you can use for your advantage.
Serious Sam: The Second Encounter isn't just more of the same, it's really one of the best sequels ever developed in the gaming history. Everything that was great in 'The First Encounter' is now superb in this one. The levels are more balanced, considering all the possible aspects: Weapons, ammo amount, number of enemies (and where/when they appear), size of the level and secrets. And, although the engine still the same, the graphics look better than the first Serious Sam, because of the new textures that allows much more varied environments. As you may already know, the events in 'The First Encounter' occurs only in the ancient Egypt (becomes a little boring over the time), while in 'The Second Encounter' you'll have medieval towns, Mayan buildings, Aztec monuments, snow fields etc.
There're also some new weapons: The chainsaw, the flamethrower and the sniper rifle. The first one you find pretty quickly, it's great to deal with melee monsters. The flamethrower is especially effective when used in small rooms or against large masses of weak enemies. But it's the sniper rifle that takes the scene, because it's effective at long distances and can also kill most enemies with just one shot (personally, I found this the most important thing).
Another great feature in Serious Sam: The Second Encounter is the sound. Although all games must have good sound effects, you'll find that, in this particular game, it's crucial. You can recognize your enemies without really looking at them. This is important, especially when predicting dangerous monsters like the bull or the kamikaze, as well knowing their relative position. The music is amazing, and just increases the adrenaline when facing the hordes of monsters. When you're not fighting, you'll hear the standard level sound track.
You can choose the difficult setting before starting a game, and it's probably one of the most important choices that you'll make in the entire game. If you choose an easy one, you'll find Serious Sam a little boring; now, if you choose a hard one, you can even stop playing because it's too punishing. So, Serious Sam: The Second Encounter is very well-balanced, and if you don't want any problems, set the difficult to 'Normal' (if you're familiar with FPS gaming). No matter the choice you made, ammunition isn't really a problem (although health can be) in any of the difficulties. This is good because it allows you to try any weapon you want, and study the most suitable one for a given situation.
Talking about strategy, Sam Stone has as his disposal a useful tool called NETRICSA. It shows information about monsters you have beaten, weapons acquired, important places and general information about the actual level. It not only gives story, but also valuable tips that definitely change your experience while playing. If a certain weapon is good against a type of monster, you'll know by just reading the info in the NETRICSA. Of course, isn't always an explicit hint, you'll have to figure out what's the best weapon considering the stated weakness. For example, if a weak monster is known by attacking in groups, then you'll probably want to use an area effect weapon (Rocket Launcher, for example) when they're all together, or a weapon with a good rate of fire (Minigun) if they're too distant.
Now, imagine everything that I already said about Serious Sam, but playing with another players. Sure the old Deathmatch is great, but it's the Cooperative mode that really shines here, in my opinion. Killing the Mental hordes with some friends (or with unknown people from the internet) is incredibly amazing. If you're a standard player, you can even consider setting a higher difficult level for more fun. Not to mention that the epic battles against the giant-like bosses will become even more epic in Multiplayer games.
There're just a little more than a dozen of levels in Serious Sam: The Second Encounter, but they're all big and each one is unique. You'll spend good hours while beating those ones, especially if you decide to search for all secrets (some are important, while others are just for fun). I like to say that 'The Second Encounter' offers a dozen of limitless hours of gameplay, considering how excellent this game is.