
Probably quite a bit.
However, without Observatory, you cannot train troops because you cannot research them. No Observatory means no sketches and no map coordinates means no trading.
You can get some troops for free and via non-PvP tournaments (Exp, Upgrade, Prizes, even Brawls), but I imagine that it would be too slow progress to make it worth it if you're trying to built up hidden and then pop up as a strong heaven. I could be wrong, though. Never tried it.
Of course, you could buy millions of rubies and end up with significant force of ships that come with those packs plus buying some at global discounts, but that'd be quite pricey, and likely not worth it.