Make CPT fields mandatory

[c3c]

Some fields should be mandatory (eg. event start /endtime)...
How to enforce this? (Preferrably a server-side solution).

Currently no checks are being done. Meaning there are errors on the browse pages.
There are some plugins that handle this: http://wordpress.org/plugins/mandatory-fields/installation/

Related:
http://wordpress.org/support/topic/how-do-i-require-a-title-for-posts
http://wordpress.org/support/topic/require-post-title-prevent-posts-with-no-title