Unity was infamously broken on launch, mainly due to the horrible face glitches. It was the first game on the new gen consoles, featuring improved graphics, animations, and lighting model. The city is densely populated and features massive crowds in Paris during the French Revolution. There's some variety to the different districts but since it takes place in a city, it's not as varied as Assassin's Creed 3 which had woodlands, mountain regions, forts and cities.
You play as Arno who is even more agile. There's some new parkour moves like the spin vault, and many downward animations for quickly scaling back down to the ground. Some of the leaps can seem a bit floaty and unrealistic though. Many buildings have open windows that you can go in which offers an alternate route, a hiding place, or can contain chests. Sometimes these windows are hard to move into, even though the game says to simply press the left trigger; sometimes pressing the right trigger gave better results, especially when approaching the window from below. When initiating climbing from the ground, I sometimes climbed the wrong thing, and sometimes Arno seemed unresponsive when you wanted to drop back down (often leaving you awkwardly perched on a lamppost whilst the guards surrounded you. I noticed that when you hold the right trigger, the controls show "back eject" but it will ignore you unless you are in a specific position and I found cases where you can actually perform it very rare.
There's plenty of things to distract you as you navigate the streets. there's events where you have to react quickly such as 'kill the criminals' that are harassing a citizen, or 'catch the thief' who will be fleeing the scene of the crime. There's other quests that I think are infinite like assassinating a particular target. There's items to collect and plenty of treasure Chests.
After certain story missions, you are told to run and jump through a portal. This transports you to some alternate time periods for brief parkour sections, featuring the likes of the Eiffel tower, or the statue of liberty.
There's some murder investigations which are pretty simplistic but feature some interesting stories. You use your eagle vision to find a few clues and evidence, often uncovering a new location to visit for more clues. You can then accuse your suspect. Although you can falsely accuse someone and get to carry on accusing others, you get lower rewards.
Unity also introduced multiplayer missions which I didn't try.
There are standard chests that you can open, and others that require a lockpicking mini game in which there are 3 difficulties. You can only carry 5 lockpicks without certain equipment that allow you to carry more. You need to use the perk system to unlock the higher level chests. Well, you can play the mini game but the timing is incredibly difficult without using the perks.
There are a few buildings you can “renovate” (purchase for a small fee) which gives you money over time. They are very cheap, and it's still simplistic and not mandatory.
You purchase new weapons and armour in a menu rather than a shop, which is weird. It would have been more organic to place shops around the city.
Some of the missions have requirements like "assassinate undetected" but given the large crowds - that wouldn't be possible, but you only need to be undetected by your enemies and not civilians. Killing in front of civilians can cause them to flee or fall over in panic.
The reputation system has been removed so guards quickly forget you have massacred them. Although it was a simplistic mechanic, I liked the fact that guards noticed you and were more aggressive if you weren't careful. This game seems more low stakes and is more lenient if you take an aggressive approach.
Without good armour, you can't take many hits, especially from the stronger enemies. The enemies are assigned a level, and if the enemies are much higher level than you, then they can quickly take you down. Not only do you need to keep an eye on surrounding enemies, but other enemies have guns so attack from range. You can't use enemies as shields to block gun fire like you could in AC3 which is a strange removal. Running through crowds seemed like a perfect option but bullets seem to reach you through the crowd, so that was a missed opportunity to implement.
Sneaking into a heavily guarded building or stronghold and carrying out your assassinations is extremely satisfying. You can snap to cover which gives you an option to assassinate from cover or use it to take a stealthy approach. You can press a button to manually sneak, then move between cover. There's a few ranged weapons like guns and the phantom blade and berserk blade (the latter causes guards to attack their friends). The stealth aspects aren't usually that important though, since if guards are alerted, you can often just revert to the brute force approach without consequences. There are a few missions where the guards are so numerous that alerting them will cause them to swarm you, but you can get away with it for the majority of the game.
There's other tools like a cherry bomb which is supposed to cause a distraction but mainly seemed to turn guards heads for a few seconds and they generally didn't seem to care. Smoke bombs can stun and reduce visibility. This can allow you to get some easy cheap kills.
When it comes to the assassinations in the main “Sequence” missions, you are encouraged to explore to find alternate routes into the buildings and have some sub objectives like stealing keys, or creating distractions. You are also given an objective which gives you extra rewards if you complete it. These can be to remain completely undetected, having a kill restriction or finding a secret entrance.
Dead Kings DLC has a new city that has a dark colour palette but some of the missions are tedious because you keep having to return to the cathedral and entering the crypt to progress further.
I encountered a few bugs but rarely anything infuriating. It was mainly ragdoll bugs where dead bodies will flail about. Some had NPC's frozen in place. I did get stuck on one occasion after dropping from above, with Arno rapidly flailing his arms. One mission I had to reload a checkpoint because my companion was stuck in a combat pose instead of running to the next area.
The dialogue sounds a bit too English with their accents. Some side missions lack dialog with Arno remaining rather quiet.
Unity makes a few changes to the series, but it's pretty much more of the same. There’s a few systems and loads of objective types but every aspect is just basic and you just casually play without putting much thought into most of it. It’s a problem with the majority of these games really.