Gemfire (ロイヤルブラッド, Royal Blood) is a fantasy strategy with rough European and Celtic inspirations. It was distributed during a time when Koei was known primarily for basing its creations on historical events or famous fictional works. Gemfire is the pioneer title for the Imagination Game (Imagination Series in English) branding, or simulations with an original setting devised by developers.

Players picks one scenario and one faction; the Lankshire family are never playable but remain a viable threat to players' progress. Each faction owns their particular gem to command, and the scenario fluctuates each faction's strengths.

Ishmeria's world map is presented through numbered provinces and turns progress on a monthly basis. Domestic affairs are simplified compared to other Koei simulations. The player has four primary functions for each of their provinces:

1. Military: assemble an army to attack neighboring province. Draft soldiers, unique units, or transport troops once per turn. Armies require gold, food, and troops in order to march. Domestic affairs helps maintain these conditions for each province.
2. Domestic: cultivate or protect territory to gain morale or officer attributes. Give food to peasantry to increase troop morale or move troops to another province once per turn. Trade by buying or selling crops at high or low prices.
3. Diplomacy: request alliance, surrender, or defection from a member from another faction. Damage a neighboring province by sabotaging or plundering it. Alliances are nonaggression pacts which cannot be forged with the Lankshire family. Each action costs one turn to complete.
4. nformation: review character stats, province information, and delegate another lord or master of a province. Does not deplete turns to complete. The player can choose to search another province to determine their fifth unit yet this consumes the province's current turn to use.

Weather conditions and plagues may universally alter the provinces and players' strategies.

Only provinces adjacent to the selected ally province can be attacked. One army can be composed at a time. Each army formation requires standard four units to take the field. If the player has employed unique troops, a gem sorcerer, or a dragon, they can act as an army's fifth unit. Fifth units are considered special units and may require specific conditions to employ. They often deal greater damage or effects unavailable to normal troops. The length of their battle capacity is determined by their troop numbers and food supplies.

Battles are capture the flag sequences arranged in a screen segregated by a flat square grid. Players can choose to manually command their units or leave army commands to the AI. Certain units can build fences on the field in an attempt to impede the opposing side. Units simply move their numbered squares and challenge any foe within their attack range. Victory can be obtained by obtaining the opposing flag and/or obliterating the competition before the food supply for either side runs dry. Failing to complete either condition has both sides withdraw in a stalemate. The player may choose to forfeit if their side is losing, yet the commander risks being captured as they flee.

Prisoners can be released or ransomed after battle. If the player chooses to banish them, the officer will be permanently removed from the game. Rulers who are captured and surrender to the victor abolishes their rule; their properties and officers will be rewarded to the victor.