If you want to restrict users from modifying the design of the database, that has to be set up separately for each database by using the Security panel of the File>Database Options dialog sheet. This isn't really completely documented yet, but it is pretty simple and there is some documentation on the Managing Account Roles page.
In order to restrict access, you must lock the database to your account. Once that is done, you can set the entitlements that different types of users can have. In your case you'll want to set the minimum level for Can Modify Design to either Developer or Administrator.
It's not mandatory to also encrypt the file, but you may want to do so. Otherwise the contents of the file will be visible in file dumping tools. There's no downside to encryption other than a slight delay when opening and saving, which I believe will be imperceptible for all but very large databases.