This is the gold standard for turn-based strategy games. Everyone is familiar with the Civ series, and Civ 2 is - in many ways - the high water mark.
Civ 2 expanded the number of units and options from Civ 1 and made some major changes that greatly improved the gameplay.
First, units were given hit-points and firepower, changes that greatly reduced the number of "phalanx slays battleship" moments. Combat proceeded in rounds so more advanced and powerful units had a better chance of winning any battle.
Second, there were costs to change from one form of construction to another. Though this didn't continue on in Civ 3, it imposed a penalty for shield accumulation.
Third, it became possible for cities to simply build wealth (once capitalization was discovered), meaning that there was always something useful for your cities to do.
Though Civ 3 added a number of minor improvements to the series, Civ 2 was a huge leap forward.