As bob the moo notes, "Asterix et la surprise de Cesar" (or "Asterix Vs. Caesar" in English) is based on two of the Asterix books. The problem with this movie is that the books in question - "Asterix the Gladiator" and "Asterix the Legionnary" - are two of the best of the lot, and either one would have made a good tale on its own, but in adapting them as one movie screenwriter Pierre Tchernia didn't do either one any favours, in addition to throwing in some odd new material (check out the attendant at the sauna Obelix visits at one point - the indication that the former is... um... checking out the latter is a bit out of place among the sternly heterosexual series).
The animation is okay, but nothing to write home about; and hearing the inhabitants of the small village of indomitable Gauls speak with American accents is distracting (though not as distracting as "Asterix Conquers America," the worst of the animated movies, which had our hero speaking in the Liverpudlian tones of Craig Charles in the English release). And note the presence of Plastic Bertrand singing over the opening and end credits (insert famous Belgian joke here).
The earlier films in the series, especially "Asterix and Cleopatra" and "The Twelve Tasks of Asterix," are better; and it's a tough call on whether this or the live-action "Asterix and Obelix Take On Caesar" is preferable - in terms of bringing the characters to the screen and in terms of entertainment and competence this one is, but then again this one doesn't have Laetitia Casta...