I'm not sure if this has been mentioned yet, but mounted troops should be commonplace in a defensive strategy also.
Withdrawals or wins damage siege. If the AI loses all of its siege, it will often just attack...even if the city is + 100% with walls/castles.
Definitely viable with any civ as a tactic, but it's stronger for protective which will tend to have castles up.
If you're protective and have stone, walls+castles are REALLY CHEAP to build for an extra trade route. They're pretty much worth it just from an economic standpoint...and protective, lacking any other economic trait, should get engineering early to take advantage of the castles but moreso the trebs/pikes (with engineering, SoDs of protective xbows and pikes will protect you from pretty much anything in the field, and trebs are good against anything in cities!)
Expand, turtle up, and make use of the land! It's not like protective is the only trait protective leaders have, so make use of the other one too

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