Sono Monban, Saikyou ni Tsuki: Tsuihou sareta Bougyoryoku 9999 no Senshi, Outo no Monban toshite Musou suru

Sieg's defensive power is through the roof, and he's proven himself to be a rock-solid foundation for a heavy-hitting party, holding every last ounce of monster aggro. But that's not enough to prevent his party from kicking him right out the door! Who needs defense when you've got devastating attacks, right? Them, apparently—without their tank, the losses keep piling up! Sieg, on the other hand, lands a job as a gatekeeper at the royal capital and starts rising quickly through the ranks. Can he defend against the hordes of attackers, earn the trust of his squadron and the lovely but quirky ladies within, and untwist the problems of the besieged city—all at the same time?

(Source: J-Novel Club)