The former Castle of Heroes III wasn't changed too much in Heroes IV at the first glance, though in my opinion there are changes and the serious ones, especially in the distribution of its habitants. First, it concerns the 4th level creatures. Let's see.
4th level: Angels and Champions
Angels got the ability of resurrection, which was the trademark of Archangels from Heroes III. However, Angels learnt it but it didn't make them something powerful. The only real advantage of Angels is that they by definition can fly. Nevertheless, every plus could be a minus and vice versa.
Consider siege combat when you would be surely tempted to send your gorgeous flying Angels inside the town behind the closed gates. What would happen then? Most probably in about 5 minutes there will be only the big pile of white feathers instead of the Angels. They are alone there and extremely vulnerable against the combined attack of the town defenders. Even the use of resurrection on themselves isn't always a solution.
Therefore, in my opinion Angels aren't the pressing unit in siege combats.
Another creatures of the 4th level and from the past Heroes games that have been significantly improved in his abilities. Now Champions are really the force to be reckoned with on the battlefield.
Despite that Champions have one third lower than Angel's characteristics in damage (24-36 against 40-65) and hit points (150 against 230) they cover this disadvantage by twice as much week growth 4 Champions against 2 Angels.
However, even all this isn't the most important about Champions. The major thing is that in Heroes IV Champions have the right for the first strike. And this is extremely important.
You can try very simple experiment and put 2 Angels against 4 Champions and watch how Angels would be dissipated in a pretty feather rainfall.
From my experience, I can tell that every time when Champions were made my major attacking force, I wasn't disappointed. In every battle, both siege and ordinary, they are superb. Count on Champions and trust them.
At first glance, there are the same old Monks and Crusaders familiar to us from Heroes III. Appearance could be deceiving. In my opinion, Monks became much more powerful than even Zealots from Heroes III, when Crusaders lost their strength and became weaker.
I got an impression that they are good and reliable in both kind of combats defending against siege and ordinary. When defending town Monks can confidently stand on platforms and shoot at attacking forces. They look like one of the best shooting unit in the whole game. Should the native magic (Life Magic) be used, then Monks can highly increase their performance both in attack and defence as well as resistance to the harmful spells of the enemy.
Therefore, my advice is that if you want to have good shooting unit reliable both in attack and defence then build Monks for your 3rd level creatures.
Let's say that it isn't the best unit of the town and it is very mildly speaking about them. I don't like them. The statistics for them looks good. Speed is 7, defence 23, 65 hit points, 10-16 damage, but they perform terribly in my opinion. Perhaps they just hate me personally? Anyway, in my experience half of them were usually killed while they were moving forward to the enemy lines, their attack lacks strength despite the fact that they can attack twice. And my god, how noisy they are! More noise than use that's for sure.
Surprise, surprise Poor knights were so bankrupted that they had to enlist in army a Thing Ballista. Even necromancers animated only something that has lived before and never thought that unanimated objects could be considered as the separate army unit. However, leaving aside sarcasm this Ballista isn't that bad at all as the shooting unit, maybe just because it isn't a creature?
Ballista is placed between Crossbowmen and Monks on the battlefield. Despite lower Ballista statistic comparing to Monks: 15/16 attack-defence ratio against 20/22, it is much better than Crossbowmen who have only 9 for attack and 11 for defence.
No range and obstacle penalties are great advantages too.
Again, we remember the wisdom of pluses turning into minuses. In siege combats when a defender moves his units down from the platforms and places them at a distance from the walls, then Ballista turns into just furniture in the battle.
Therefore, it's better to use them in open field battle and when defending the town, not attacking it.
Another familiar face from Heroes III but with unfamiliar features. It was the pleasant discovery to notice that Pikemen were changed from weaklings of Heroes III to quite capable army unit in Heroes IV. The gameplay showed that they earn much higher marks now than ever before. Pikemen are excellent as both defenders and attackers. When in mass they present a considerable strength of the army and pose serious threat to the opponent.
Pikes became the long reaching weapon permitting to attack enemy at a distance even bigger than for Crusaders. With using advantages presented by natural obstacles of battlefield and right tactical decisions in your army units' deployment, Pikemen can be a great help.
In Pikemen advantage, comparing to Ballista, also could be said that they would never be useless ballast in siege combats. Therefore, Pikemen are the better of the available 2nd level creatures for Haven. I prefer them.
As usual, first level is easy. No headaches what to choose. You have two units: Squires and Crossbowmen. That's what you can build, but Heroes always has some hidden layers and under this simplicity, there are more choices than you thought.
The question is what kind of army you plan to form at the beginning of the game. The key question is who will it be Monks or Ballista. Will you need just one unit of shooters or more?
Crossbowmen aren't mighty, but they are shooters nevertheless and therefore represent the force that can longer keep enemies from approaching to your lines for hand-to-hand fight. They are especially useful on the first stages of the game in this role. In addition, they don't have any range penalty.
They don't look impressive and walk slowly (5), (probably they were cavalrymen before but without their charges, they feel uncomfortable on the ground) but they have one interesting ability stunning. The stunned enemy unit temporarily just dropped out of the battle. Isn't it useful quality in the 1st level creature? Moreover, they have good week growth volume 21 squires every week. The squire school is working three shifts per day!
Just one thing to keep in mind stunning doesn't work always and not on everyone. Nevertheless, Squires have it and use it, at least occasionally.
Proceeding from the above comparative analysis of the 1st level creatures, you can see that both units have pluses and minuses. It is for you to decide whom you'll use more heavily. As for me, I usually took Crossbowmen in battle in order to ease my initial movements around the town.
All in all Haven of Heroes IV is significantly weaker than Castle from Heroes III because of many reasons and with changes in magic not the last among them.
Therefore, playing strategy for Haven must be completely revised for those who played Castle in Heroes III.
Here we have two classes that could be anticipated in such a town Knight and Priest. Naturally Knight is a might hero and Priest is a magic hero.
In my opinion, Knight is weaker than Priest, but pay attention that it is so in single scenarios only! In campaigns, Knight can be indeed a very good hero for you.
Priest, on the other hand, is inferior to similar heroes from other towns. First, it is connected to the specifics of the available magic and the fact that it is harder to develop on the first stages of the game. Later in the game, you must check that your Priest would have additional magical schools, otherwise it could be problems again.
Town magic (School of Life Magic)
Life magic is the perfect example of unity of opposite. It is both strong and weak at the same time. If you'll remember what spells could be received in the magic towers of Castle in Heroes III, then you'll notice that powerful attacking spells were withdrawn from the Life Magic arsenal. Now it is more defence-oriented and has many healing and warding spells. In healing and restoring both health and morale, it is the best in the whole game. Both these things are actually quite important in the game. So, don't be overly distressed, guys.