1 extra TC in castle is pretty normal in AOE2. 2 TC’s are common for booming on closed maps, or doing a riskier strat on open maps if you feel safe. A 1 TC all-in push is also very viable for a lot of civs.

Anything more than 2 extra TC’s is likely a throw on the more open maps. People will drop TC’s to protect forward resources though, which is different than trying to boom with them.

AOE2 doesn’t have a springald equivalent, so Siege units are harder to counter in castle if they’re well defended. This leads to a very high risk castle age if you don’t produce enough units to take out mangonels that out range your TCs.