Hey Bombers! Long time lurker - first time poster! Hoping for a hand-out here... (Also, I apologize - English is not my native language). Anyhoo...

I've gotten to The Lost Bastille. I've lit 2 bonfires, both The Holding Cells - from sea, and The Tower Apart - from land.

I can get from one of those to the other, and have so far only found a Pharros' Lockstone-thingymajig; otherwise I'm stuck. Is that the key to my onwards journey? Because in that case, I just guess I need to go grind some other areas in search of a lockstone.

So, do I need to grind on in this particular area, or did I miss some crucial object earlier on?

Thank you in advance :)

You've missed an area. It's in a tower behind a closed door. Easiest way to get there is from Tower Apart.

@ares42is correct. Regarding the lockstone, that's an optional area but totally worth going through.

Thanks a lot, Gents!

Kinda typical Dark Souls this...
"Should I grind this area or that one to progress?"
- Both.

The key to continuing is simply to beat the 1st boss. You mustn't have gotten to them, or you'd be mentioning the bonfire immediately after them. That boss is hard to miss. Just go inside the jailhouse with a ton of swordsmen inside.

Lockstone areas are always optional. There are 2 that I know of in this zone. The first one doesn't have great loot, but will open up a moderately useful shortcut later on. The other lockstone area after the boss leads to a new zone, but again it's optional. If you only have one lockstone, I'd say to save it for after the boss.

Also, did you miss that important bonfire next to the blacksmith? Near the boss building, go to that big crack in the wall and roll an explosive barrel into it.