i did this last year, we had a crown jewel treasure hunt in the garden (plastic gold coins from wilko's), and made crowns and decorated (you can get templates from baker ross but I just cut up card) and all the children dressed up as princesses and princes. You can get huge cardboard castles which can be painted online which I thought about too late. We also had a crown shaped pinata.
oh and plastic champagne flutes with tinsel on for drinks, silver platters and a silver cupcake stand for the royal table!
Pinata - shape of a dragon (keep kids well back), send them on a Quest (treasure hunt to collect items ) around the garden, Bouncy castle. Castle shaped cake or a princess (or use figures form Playmobil !)
its just a flat round cake, the turrets are white icing set around cardboard tubes and everything is cut out of roll out icing or piped on - it was actually not very hard, def easier than the bloody barbie shaped one I did the year before.
i got our pinata from woolworths online, they have a party shop online and were very quick to deliver.
IIRC, we basically did standard party games with a princess/pirate twist - musical castles (printed on paper, instead of musical chairs), pass the parcel (not themed!) and some other craft activity which escapes me just now.
For a knights party I did once get really cool giant cardboard castles which they coloured/decorated and used in games - those would work for princesses too?
Key rule for knights or pirates - all weapons to be checked in at the door - for knights party we then gave them all inflatable swords and let them riot after tea - it was chaos, but they absolutely loved it.