The biggest change between OMD1 and 2 is the addition of co-operative play. It has been integrated to such an extent that playing solo has been slightly compromised, there are conversations with one side missing and the new levels often have enemies fighting fairly far apart and there are significantly fewer teleporters than in OMD1.
Most of the old traps and enemies return along with a generous helping of new ones. The game itself feels very much to be more of the same. As a bonus, those who own the first game get a good chunk of those levels available for solo and coop play in Orcs Must Die 2. Allowing you to tackle The Finale with two players, vastly improving that level in my opinion.
Robot Entertainment also changed up the skull and upgrade systems. The 5 skull scoring system still stands, but now there are bonus skulls. Even if you manage to leak that one kobold, it is still worth continuing through to the end, because finishing a level will give you at least one bonus skull, if not more, you get bonus skulls for killing lots of orcs, for not using traps, for not using mana, for not taking any damage, for not leaking any enemies. These can really add up, on Nightmare I can fairly reliably get 15 bonus skulls.
This helps with the new upgrade system, rather than the single step upgrade system from OMD1, each trap and weapon in OMD2 has a bunch of upgrades. A main path with 3 levels, a choice of two unique upgrades (than can be freely swapped between outside of levels) and possibly special upgrades. Combined with some traps having an initial unlock cost it can cost upwards of 40 skulls to unlock and fully upgrade a trap or weapon.
This means that whilst you earn a lot of skulls you will be spending them like water, even having gone through the normal campaign, the classic levels on normal, and some of nightmare and skull farming on Chasm I still have plenty of traps, weapons and trinkets to unlock and upgrade.
Trinkets are a new element, they take up a slot and provide a passive and an active benefit. Increasing health and regeneration, or giving enemies a chance to drop coins, decreasing trap reset time. They don't directly kill orcs but I find that I almost always spend 1 to 2 slots on them because the passive effects are so damn useful.
As a result I easily have 5 times the number of hours in Orcs Must Die 2 compared to the first, the cap of 2 players is both annoying but useful, because I only need to find a single buddy willing to play to get a game going I get games a bit more frequently, it just feels like we are excluding any other friends who go online during the game.
All in all, Orcs Must Die 2 is a great game and I recommend that you get it if anything I have said has interested you. You also should pick up Orcs Must Die 1, but only because that unlocks the classic levels in OMD2. They are currently 75% off on steam, they even have a demo if you just want to check it out.
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