I just want to confirm that every turn a player takes has a combat phase, even if the active player does not declare any attackers?
Specifically, in a multiplayer game with 6 players (me and 5 other players), if I have a Citadel Siege on the battlefield and I chose "Dragons" when it entered the battlefield, on each of my opponent's turns, will I be able to tap a creature that player controls (as long as they control one that I can target)? Or can I only tap their creature if they "move to combat" on their turns? Asking for a friend (Gideon's Avenger).
Yes. There is always an Untap Step, Draw Step, Main Phase (both Pre- and Post-Combat), Beginning of Combat Step, Declare Attackers Step, End of Combat Step, End Step, and Cleanup Step (barring any effects that affect these of course). Magic is not clairvoyant; it must give players a Declare Attackers Step all the time for them to even get the choice to not attack. And, because Magic operates on a system of priority, players can't skip steps just because they don't want to do anything. Others still might.
Only if there are no Attackers Declared are the Declare Blockers Step and Combat Damage Step skipped (and only if something with First or Double Strike is in combat will the First Strike Damage step be created). These are the only 3 steps that will sometimes happen and sometimes not happen. Anything else always happens whether players want to do anything in those steps/phases or not.
Rule 500.1: A turn consists of five phases, in this order: beginning, precombat main, combat, postcombat main, and ending. Each of these phases takes place every turn, even if nothing happens during the phase. (…)
Specifically, in a multiplayer game with 6 players (me and 5 other players), if I have a Citadel Siege on the battlefield and I chose "Dragons" when it entered the battlefield, on each of my opponent's turns, will I be able to tap a creature that player controls (as long as they control one that I can target)? Or can I only tap their creature if they "move to combat" on their turns? Asking for a friend (Gideon's Avenger).
Thanks.
Only if there are no Attackers Declared are the Declare Blockers Step and Combat Damage Step skipped (and only if something with First or Double Strike is in combat will the First Strike Damage step be created). These are the only 3 steps that will sometimes happen and sometimes not happen. Anything else always happens whether players want to do anything in those steps/phases or not.
RULES OF MAGIC :
http://magic.wizards.com/en/game-info/gameplay/rules-and-formats/rules