Leon only intended to get close to the castle on the mountain peak to take a look, not to do anything bad, especially not burglary. So, he chose to avoid sensitive matters that might alarm the castle owner, opting to approach the castle in human form by foot.
If the owner of the castle was polite, it would merely be a regular visit. If the owner was aloof, practicing a 'strangers not welcome' policy, he wouldn't push any further. If the owner had malicious intent, then he would have a reason to make a big fuss.
Transforming into his Dragon Form would involve just a casual flap of his wings, while his human form required quite a bit more effort. But since he wasn't in any rush, Leon was perfectly unhurried.
