Mindy pointed out the four great areas to easily save some money on.
Going with outside floral can really save you money compared to what Disney offers, as can going with an outside DJ (our DJ was less than half of what Disney charges even after the power fees and he was absolutely outstanding). Do as much DIY for decor that you can yourself can really help too (but don't fall down the expensive easy hole). We bought tons of old books (usually for less than a dollar) to use as centerpieces at our wedding, and now I have a large collection of fairytales to share with our children. If you're willing to be flexible on time of day, lunch weddings tend to be less expensive than dinner weddings because the food and beverage minimum per person is lower. Keeping your guest count low is really the key to keeping costs down for a Disneyland wedding. The food and beverage adds up fast.
Also keep in mind that the cost for the park shoot is in addition to the event minimum. So if your event minimum is 15k, the cost of the castle shoot is on top of that minimum. If you're preparing to sign your contract, see if you can get their current preferred vendor list for photo/video for the castle shoot since Mindy is correct that only the preferred vendors can do the in-park shoots. However, you do not have to use that vendor for your wedding day, so you can book someone who fits in your budget better for the wedding day if it works out that way. The package with our photographers included an engagement shoot, which they were willing to apply for our morning after castle shoot, which helped keep costs down for us.
Another trick to help keep costs down include re-using your ceremony floral/decor for the reception space. Also, the more items outside of the Disney wedding you save money on, the more money you have for the wedding itself. For example, we did our own invitations using an at home kit from Michael's for less than $50 after coupons and we created over 100 invites. Shop around for hair and makeup. if you're going to stay on property, consider staying at Paradise Pier since it is less expensive. The little cuts you make like that add up quickly so you have more money for the wedding itself.
If you're doing 4 characters, pick the pairs. Disney charges less for the pairs than 4 individual characters.