The crossbow will let you create a way into the Princess tower. Get the rope from the water wheel at the mill and tie it to an arrow. Aim up to the left and shoot until you hit the door.
If you tie a rope on one and shoot it up and to the left, you might be able to reach the Princess tower on the castle. You can't jump to it.
The Binary Bard's castle is on a large planetoid in the asteroid belt, near the coordinates X-11 Y-84. (The asteroids can damage your ship unless you dodge or shoot them.)
on astro-knights.
One of the robot attackers is in the castle dungeon. The alien ships attack you in outer space when you are near the Fire Planet and the Jungle Planet. Dodge their shots and shoot back, and land as soon as you can.
Yes. Your spaceship is parked atop the castle. But all you can do in space is shoot aliens, shoot asteroids, or hop on the plants and Phoenix eggs. Everything else is completed, and the boss locations are all dead ends.
There are no fish on Astro Knights Island. There are Space Sharks around the Ice Planet at X-73 Y-83. To defeat them, shoot them and lure them to the Black Hole at X-85 Y-47. (see related question)
Dodge them or shoot -- click your mouse to shoot, and watch out for pieces. (If you take a Space Shark to the Asteroids, he swims right through them!)
You shoot them
To reach the left-side tower of the castle (the Princess room), you need to use the rope from the Mill. Tie it to the arrow on the giant crossbow and shoot it left and up to hit the tower door. (You get many tries.)
get the rope from the wodden thing in the mud next to the mill, then put the rope on the arrow, and shoot it towards the castle.
You have to hold down the mouse and when the mouth opens you shoot. You need to do it three times.
It is on the outside ramparts of the castle. Get the rope on the wheel beside the windmill, and tie it to one of the big arrows. Shoot up and left to hit the door in the disconnected Princess tower. (You get more than one shot.)