Confused whether to add town centers at Castle Age or NOT? I will answer your question in two paragraphs with priceless tips.
Top Three Situations Where You Should Stay On 1 TC
(1) You have more or equal army of your opponent
A- e.g. You got 20+ crossbows: get a university then research ballistics. Image below shows all crucial upgrades you need.
B- e.g. you got 20+ farmers/12+ gold miners to support nonstop Knights (called a mini boom) => Rather add a forward siege and ram/mango his military structures. Check important upgrades for knights below.
(2) When an Enemy has a mass of crossbows or Knights or whatever. Meantime you barely got any army, you must instantly build a siege. Queue 2 mangonels to defend against crossbows. Or build a monastery against Knights.
Keep the production of whatever military you are building until you have enough only then start thinking about adding tcs (tcs after 24 mins are considered late so watch out for time).
(3) Against an opponent that goes 1 TC all in you can and should do the same until you are sure he is no threat.
Top Three Situations Where You Should Build 2 Extra TCs
(1) You did a drush fc on your opponent and you can afford both army and tcs (add one tc at a time to keep your eco balanced for military as well)
(2) You are attacking him at his base and he’s busy dealing with you. Take another advantage and tcs boom behind that. (but keep your army alive as long as possible)
(3) You just took an advantage by killing all his army before castle or in early castle so you might as well take another advantage
Civilizations That Get Away With Extra Town Centers?Why?
Britons Why? 1 TC costs 150 wood & 100 stone.
Celts Why? With Their wood gathering bonus, Celts often float extra wood.
Burmese Why? The free Wood upgrades let them stockpile wood faster all over the next 3 ages.
Byzantines Why? By default one of the best defensive civilization in the game due to access to cheaper defenses and structures get bonus HP every age.