If I skip a secret level and continue with the main ones, will I be able to play them later?

Thanks.

Yes, except for the one that only shows up during the Full Moon; the Full Moon level is available during any full moon, and only during a full moon, with the game using your system clock to determine whether there is a full moon.

Also, be advised that some of the bonus levels are broken, as in "cannot be completed, or provides no benefit". Other bonus levels can be used to make the next mission much easier via Transfer Creature and Level Up.

But the moon level can also be played as often as you want, provided there's a full moon out.
If you play KeeperFX, all the secret levels are fixed. And also, on level 18 there's supposed to be a dungeon-special to unlock the final secret level, but that's been forgotten to be placed so the final secret level can ONLY be played in KeeperFX.
They have also been reordered. In the first few levels you cannot train demon spawn past lvl10, and than in the last level that is the case you can unlock the secret level where you start with a demon spawn and get many level-up-specials to train it into a dragon. After that you can train demon spawn into dragons on the campaign as well.
This is the level that could not be accessed in DK gold at all.

Thanks all. Yeah, that was the point. I completed a level with a Vampire10 transfer, played a secret level which rewarded me with a level 10 Vampire, making the whole thing useless.

But if I can save those secret levels and play them anytime then all is good.

(Also, *sigh* I'm repeating myself but I just can't get into this game. I loved all of the Bullfrog classics so far, whether I played them when they came out or recently from GOG, and I'd really want to love this one too, because it seems like a really great game, but I just can't...)

It isn't really a matter of "saving" them, because you can do them repeatedly. Want to start every stage with a level 10 creature? Once unlocked, the secret levels have you covered.

You're right, but I want to play every secret level (those that are not broken) once. But not more than once.
Since I'll be playing the secret level anyway, might as well play it where it counts.

It's actually just the last 3 levels where you don't have access to a transfer- special. So until then simply always look for it before you complete the level.

I actually don't like the secret levels at all, the two imp-killing ones are acceptable I guess. Ironically, I think the missing one is the best one.