First, you should know about the destructive power of the sappers. They deal 400 damage to every unit and building next to them. That makes them the strongest unit in Warcraft II in terms of damage - they have only one strike, though. Sappers are even stronger on sea levels - transports allow deadly "hit and fade" operations, and human players can even use invisible transports.
Because a sapper damages every building within a 3 x 3 area, you should not place your building to close to each other. The image to the right shows the typical town of a newbie: he places his farms next to the previous ones in an arbitrary order. This formation cannot only block your units on their way north (it's a tiny "maze" for them), but also be destroyed by a single sapper.
If you don't believe me, watch this! One sapper, costing 750 gold and 250 lumber, takes out four farms that cost 2000 gold and 1000 lumber. The player who lost the farms will certainly lose the game, not only due to the lack of resources, but also because he won't be able to train new units for quite a while - he'll have to build four new farms first.
If you ever see some buildings standing that close together, take them out! This can win the game for you.
The next picture shows you how to build instead. Always leave the space of a farm between the buildings. Thus, your units will be able to travel through your town swiftly, and sappers won't be such a threat. Of course, if you are using a special wallin strategy, you will be forced to place the buildings next to each other. To disable sappers from destroying your buildings at once, just build walls at the crucial spots to prevent them from attacking there. (It may need some experience to see where these spots are.)
Sappers are rather expensive units, being very wood-intensive. They are therefore especially strong on maps that lack gold. But you should know how to use them. Some players even go for sappers only, which is risky but may be devastating.
First, you'll need at least two alchemists. Use one to produce flyers, the other one for sappers. Unless you are only planning a few surprise attacks, you'll need even more alchemists later since the sappers take very much time to be trained. I'd use about four to six alchemists, depending on the resources available.
Second, you won't use sappers against units (except catapults or perhaps mages) unless more than two units are standing too close together in a nice formation - like in the picture :-)
A catapult is always a nice target because it cannot escape and costs more than a sapper. Don't waste your sappers against two knights, you may miss one because they will charge you, and a knight needs much less time to be trained than a sapper, so your gain will be small unless gold is scarce. Always watch and guide your sappers! You don't want them to take the wrong way or run into the enemy.
Use sappers to take out key buildings and badly built formations (see above)! Key buildings are namely those that exist only once, e. g. a blacksmith. On low resources and land only levels, taking out a blacksmith with two sappers is always bad for the enemy. He won't be able to build knights or ogres for a while and he'll lose 100 oil. Because you have only 200 oil left after upgrading to castle, doing this three times will leave your opponent without knights! Of course he'll hardly let you do that.
Other good targets are mage towers, alchemists, and, of course, towers. Towers are meant to be taken out with sappers ;-)
A tower costs much more than a sapper and may prevent you from launching attacks. After the towers are gone, attack immediately and your enemy's defense will probably be too weak since he had relied on his towers. Be careful against guard towers, though, because if several guard towers are standing close together, they will kill your sappers before they can reach their target. In those situations, it's time for magic or catapults to do the job.
Always attack the buildings of which the loss will hurt your enemy the most. For example, losing one out of seven barracks doesn't hurt much; instead, losing one of two mage towers does. If a player negliges a certain building type, go for it! Some players only build as many farms as they need, no "backup" farms. This is good at the beginning where speed is everything, but later in the game, if you take out two farms or more with your sappers, he'll be delayed very much. It's time to attack him then!
Attacking a castle with sappers is only realistic in water maps where you can unload a transport of sappers near the castle. On land maps, the sappers had to walk through the entire enemy town, and they would hardly survive that.
A castle has 1600 hit points, so you think four sappers are enough to do the job, right? Well, if you're lucky, that's true. However, if only one sapper fails, the other three are wasted. And even if all of them reach their target, the opponent may have enough time to start repairing his castle, and then it will still have a few hit points left after your attack. So always have another sapper ready (or an attack force). You should use a transport full of sappers. Even then, if the other player has a good recon, your attack may fail. If you're successful, though, the other player will be thrown back a lot unless he has already lots of buildings that need a castle to be build (they will remain operational).
This section is for beginners only. The number of sappers need is normally the number of the target's hit points divided by 400, perhaps plus one being ready to attack if the hit points of the building are a multiple of 400.
The shipyard (and the foundry) can only be attacked from one position so it's easily defended against sappers.
Good units to prevent sappers from reaching your town are: