The Landsknecht is the unique unit of the Holy Roman Empire.
Strategy
Compared to the Pikeman, which the Landsknecht replaces, it is twice as effective against Melee units.
Using Landsknecht
Landskecht are cheaper than Macemen and at face value, are an even match. However, they're weak at attacking cities, due to the lack of a bonus against ranged units. Therefore, they work best in defense - which helps compliment the low-maintenance defensive city-building nature of the Holy Roman Empire.
Mass-producing Pikemen to defend against would-be pillagers could be a huge waste of time normally, but with the huge bonus of Landsknecht, they can be put to other purposes, such as defending a city-attacking stack.
Against Landsknecht
Engineering is a fairly awkward technology to pick up, due to its reliance on high-cost previous technologies. As such, a Holy Roman Empire player needs a good research base to make these units really shine. Take out the commerce-based cities earlier on, and you delay these units that little bit further.
They may be effective against Melee and Mounted units, but they have no advantage against Archery units. Use Crossbowmen against them to hit their weak spot.
Civilopedia Entry
The Landsknecht were a Renaissance style of German mercenary (although the individual members came from all over Europe) which were renowned for employing a tactic known as the "pike square." In the pike square, each soldier would be equipped with an eighteen foot long pike, and would stand in a tight, and sometimes hollow, box formation. If attacked from any side, the pikemen would simply lower their weapons and brace their pikes into the ground. This tactic was devastating to enemy cavalry and infantry. Surpassing the Swiss mercenaries organizations on which they based their tactics, the Landsknecht eventually became one of the most successful style of mercenary in Europe due to their willingness to integrate newer technology such as arquebuses and gunpowder artillery into their military formations.