Lowering the drawbridge allows you to collect the very handy Ring of Binding (Sets maximum HP reduction from death to 20%, instead of 50%), an Old Radiant Life Gem and a Human Effigy from a chest and corpse. The Mist Door here leads you to a fight against the Old Dragonslayer Mini boss.
Note: Outside the mist door is a NPC summon point that can make your life a lot easier during this battle. The NPC warrior can distract the Old Dragonslayer, giving you time to heal or get to a safer position.
Mini Boss: Old Dragonslayer
Personal Death Count: 30+
Charge Thrust - Old Dragonslayer will crouch down and move his spear arm behind him before racing towards you position with a gust of wind and a merciless thrusting jab. Rolling to the right is the best way to avoid this. Can be confused with his jump attack so be careful about which one it is before rolling away.
Jump Thrust - Old Dragonslayer will quickly jump into the air, moving his spear arm behind him and then thrust down to the ground towards your position. Rolling backwards works well to avoid this attack, just make sure you aren't backed into a wall.
Jump Slam - This is the attack that can be confused with the charge thrust. The Old Dragonslayer will quickly crouch (a gust of wind will appear beneath him) and leap into the air before slamming his spear down on your position. Timing is crucial here as if you dodge too early he will adjust his aim before attacking. You need to roll just as his spear is about to slam down, so he cannot readjust. Rolling to the right at the correct time will get you to safety.
Jab and Up Thrust - This is one of the Old Dragonslayer's slowest and easiest to dodge attacks. He will jab towards you and then perform a second thrust upwards into the air. If you rolled backwards from the first jab successfully, you should have more than enough time to land a hit or heal if you need to while he performs the upward thrust.
Jab/ and Swing - The Old Dragonslayer quickly pulls his spear arm back and jabs it towards you, this may or may not be followed by a sweeping swing of his spear in an arc in front of him. Roll backwards to avoid this attack.
Dark Triple Shot - The Old Dragonslayer will charge dark magic in his spear for a second or two and then fire three projectiles at you. They are relatively easy to dodge and you should be backed up far enough from him during the charge up to get a free hit in after rolling if you are a ranged character.
Dark Projectile - The Old Dragonslayer will charge dark magic in his spear quite quickly (a second or so) and then fire a single quick projectile at you. Roll to the side to avoid it.
Wow, this guy (like the Pursuer before him) is far tougher than his relatively small size would have you believe. He attacks rapidly, has a large number of attacks and only a few of his recovery animations are long enough to allow you to heal. If you brought the summoned NPC into this battle, it is still difficult but much more manageable. The Old Dragonslayer (like every boss so far) is right handed and if you have been getting used to dodging to the right side of a boss, it will pay major dividends here as roll timing and direction are crucial here. As soon as you enter the cathedral he will charge thrust from the other end at you, covering the whole room. You can get off two shots as a caster before he reaches you (free damage).
You must be consistent in dodging this boss as the only real heal opportunities are after his jab and up thrust attack and his jump slam attacks. Damaging him in melee is a dangerous game of hit and run, rolling to the right or away from his attacks and circling to his right side to attack. He reacts quite fast, so make sure to always be ready to roll away after attacking. Casters will have to try to get a spell off immediately after dodging one of his attacks. Only one, mind. Any more and he will perform his next attack, which greedy casters will be unable to dodge. After enough tries and memorization, you will eventually wear him down and defeat him. After winning you will be rewarded with an Old Dragonslayer Soul and the Old Leo Ring (increases damage of thrust attacks).
After clearing out the cathedral you will be able to reach the balcony beyond. Inside the chests out here is a Cracked Blue Eye Orb, a Heide Knight Iron Mask and a Tower Shield. At the edge of the balcony is a Knight of Blue who will offer you a covenant with the Sentinels of Blue if you have been summoned to another players world and helped them defeat the boss. You also can defeat him in battle to obtain the Blue Knight's Halberd. A nearby staircase leads down to a bonfire. Now that the cathedral is done, use the bonfire to teleport back to the entrance to No-Man's Wharf (which you should have activated in Heilde's Tower of Flame section) as that is our next destination.