27%
Assuming all pre-requisite buildings were already completed for level 23 mines, level 27 mines, Wall and watchtower. Also assuming wall and watchtower were at level 26 prior to upgrade (thus 3% and 1% rather than 43% and 16%, respectively). Cradle adds 2%, Unusual Warrior Helmet adds 9%. Ice boots add 0%, Inlaid belt adds 9%. Usual White Robe adds 0%. Brodir’s flail would add 15% but since not level 50 hero, he can’t equip it, so adds 0%. 4x Beast Shell runes adds 36% but only have 2 legendary items to add to so only 18%. Lands of Strong Shields adds 60% and Buffoon repute -75%. Also, assuming VIP level 25 was already established before the increase calculation, otherwise would add another 23% (from level 1 to 25). Finally, assumed SH level 2 was already established prior to calculation, otherwise, defense would be +8% higher.