Or merging.

As I left Treva's Watch, via the Escape Tunnel, after the Striking the Heart assassination I immediately encountered a group of NPCs, some Lord and 2 bodyguards, who asked me to "retake the castle". The first objective for their quest, Infiltration, is to open the gate... obviously I had already opened the gate as I found a way to jump into the courtyard. I completed this quest by actually closing the gate, then reopening it so these NPCs could enter. Everyone inside was already dead... and where all Silver Hands.

This Lord was not there when i initially found my way into the courtyard and opened the gate.

I guess players where meant to bump into the Infiltration quest while initially gaining access for Striking the Heart. However, because of the backtracking nature of quests coupled with exploration I had opened the gate before receiving either quest. The sequence of these quests is too easy to screw up. One obvious solution would be to wrap Infiltration into Striking the Heart, leaving only one quest.

I completed the Infiltration quest not too long ago, just randomly exploring. Its one of those...find it and do it quests, although I think it can be assigned as a work or bounty from Riften (I got that impression from the NPC's dialogue during the quest, sometihng about a posted reward). I don't know what Striking the Heart quest is for, but it sounds like a number of other quest events I've had where an important side quest will take place at a location/dungeon, and then once you leave it resets to its normal thing. For example. the normal thing for Treva's Watch is that lord guy, the Infiltration quest, and bandits, not Silver Hand. The locations just seems to be borrowed. The quests are not interrelated, I believe.

Striking the Heart is a quest for The Circle/Companions. I did essentially what you did- I first found Treva's Watch while exploring. There where a few hostile NPC's- IIRC there were Silver Hand as I likely already met the requirements for them to be hostile- in the courtyard, which led me to look for a way in. I climbed up within ~10 meters and in plain sight from where the Lord spawns... and he wasn't there. From what you say though he could/should have been there.

The way I experienced it made no sense.

Resurrecting this thread as I'm running into a similar problem now. I found this Lord and his bodyguard before I went to Riften or followed through with the Companion quest. The fort was full of bandits and I infiltrated, not through the special tunnel but over the back wall, and completely cleared the place. At the end it seemed like the quest had bugged because the Lord didn't seem to realize I had opened his gate and he wouldn't go back into his castle. He stays camped outside. He says something about searching for his daughter who wasn't in the castle. Well, it was just a side quest so I let it alone. But now I have a quest from the Circle of Companions to get something from the Silver hand in here. But that quest seems bugged; I don't get any quest markers at the location and the fort is filled with respawned bandits not Silver hand. I play on xbox. How can I fix this?