That is a big castle. I'm informed the Koopa Bros. are squatters.
Here's Koopa Bros. Fortress. This used to be ruined, but I guess the Koopa Bros. rebuilt it. It looks pretty huge, huh? And solid, too. This fortress was originally built around the same time the Bob-ombs took up residence on Pleasant Path.
On the plus side, it sounds like we've caught them on the back foot.
The Koopa Bros. Fortress theme isn't the Fortress theme from SMB3, although it does call on the riffs from time to time.
This is a Bob-omb. Bob-ombs are pretty temperamental, and they get steaming mad when you attack 'em. Max HP: 3, Attack Power: 1, Defense Power: 0. You'd better not jump on 'em when they're angry or they'll blow up in your face. In fact, stay away entirely. Attack 'em from a distance with items like Hammers. Or you could send in other things, like shells. If they attack when angry, they'll charge in and explode. So you'd better take care of them before they blow!
Bob-omb (Lv. 6) first appeared in Yume Kōjō: Doki Doki Panic, the licensed game that eventually became Super Mario Bros. 2 in the west, and of all the enemies from that game that made it into the main series, Bob-omb was by far the fastest and most prolific. These days, Bob-omb is as recognisable as Goomba and Koopa Troopa, if not more than some other SMB1 classics we'll meet later. In this game, their 3 HP is a bit pointless- hit them once and they get angry, and then they'll explode either when you hit them or they hit you, instantly removing them from the fight. You want Bob-omb to explode on your attacks, but that can be difficult to set up if they're not the closest enemy in the formation.
Fortunately, if you don't anger them, they won't damage you if you Guard.
Don't worry, a single Jump won't get the Bob-Omb to explode under your feet. It will explode on the next attack, even though its HP is going to 0.
Also, an angry Bob-omb has heightened resistance, if not outright immunity, to status ailments a peaceful one does not. If you've angered a Bob-omb, it's too late to play the trickster.
Mario's Hammer and Kooper's Shell Toss will blow up a Bob-omb without damaging the attacker.
That first enemy also had a key on their person. Yoink.