Castles is a strategy game with building simulation overtones that focuses on the design of medieval castles. The goal is to build an invincible interconnected system of fortresses that stretches across eight strategic Celtic lands (full campaign mode). Three-castle and single castle campaigns are also available. In a full campaign you must build a self-sustaining castle in the first territory and then withstand the inevitable attack by an enemy force. If the defense is successful, settlers will build a nearby town and trade with Celts who eventually become dependent on you for goods and services. Expansion into a neighboring province naturally follows and the process is repeated, culminating ultimately in a united kingdom.
As ruler, the player will make all detailed decisions regarding castle construction (blueprint/design, workforce, defense) through a complex system of assigning tasks to seven wage-earning specialized laborers (masons, carpenters, quarrymen, diggers, smiths, carters and common laborers) while striking an acceptable balance in the management of taxation, levies and budget outlays for labor, military and castle expenses. Castles is customizable in several areas including difficulty levels (four), type of world (real or fantasy), campaign length, castle type and diplomacy (messenger) matters. Two viewpoints are utilized (top down for blueprinting and three-quarters for building).