Most turn-based videogames use non-simultaneous turns, where one unit of one player or side moves at any time.
Simltaneous turns are more difficult as they raise lots of concurrency issues, and I will not discuss them here.
See also this blog article at Gone Gaming for a broader discussion of turn-based game mechanics.
Non-simultaneous turns come in at least 3 common flavors:
Units on each side Chunked together
1. Each side moves its units in turn. Individual units make a move/action once per turn. Examples: Advance Wars, Ancient Empires, Many Board Games.
2. Each side moves its units in turn. Individual units can interleave their moves & actions arbitrarily within the turn, changing focus in between. Examples: Civilisation, Arcadia
I see option 1 as really a lite form of 2, where the mechanics are simplified because a unit is never in a partially moved state, and there’s less need for controls to cycle between active units.
Variable Action Frequency
What Indivudual Turns allows that Chunked approaches cannot, is the ability for fast units to act more frequently than slow ones. Their turn can come up more often. I really like it, its quite realistic and better matches real time games. However, I dont think it scales well to losts of units that need to execute team maneuvres, because their individual turns are scattered through time. Hence, that mechanic tends to be used in Tactical style games featuring a small number of participants. In Strategic games like Civ, where you control many more units concurrently, having all your unit moves happen at once makes it easy to execute large plans of action.
Chunked approaches often struggle to model different unit speeds of in a meaningful way. Typically, units can move different amounts but are allowed one attack, spell or similar action per turn.
Movement Points In Arcadia
In Arcadia, we are trying to meet the challenge of variable action frequency via variable number of Movement Points (MP) allowed to each unit.
- A unit can attack, cast spells or any other action multiple times per turn, if it has any points remaning. (So MPs are really Action Points.)
- Different actions have different costs; movement in the open costs 1; a melee attack costs 3.
- If taking an action would result in a negative MP balance (eg Unit with 1MP attacks, costing 3MP), we allow that! But, we store up the debt and repay it, in effect an overdraft, so that next turn the unit will be able to do less.
Where this approach is really nice is its robustness to MP-reducing effects. For example, imagine a Slow spell reduces a unit’s MP-per-turn to 1. Without an overdraft, it would struggle to every attack, as attacks cost 3. With an overdraft however, it can attack ever 3rd turn, which feels just.