There are 27 churches in this sestiere; only Cannaregio (with 32) has more.
San Zaccaria is a must because of its drop-dead gorgeous Bellini altarpiece, but there’s much else to see including a nice gothic chapel and a crypt that you can visit if it’s not full of water.
Another favorite is San Francesco della Vigna with its beautiful Madonna (see left) by the Franciscan friar Antonio da Negroponte, another mysterious artist. This is his only known painting. If you’re only going to bat once, you might as well knock it out of the park as he did. I also love Cima da Conegliano's Baptism of Christ that's on the high altar of San Giovanni in Bragora.
San Giorgio degli Schiavoni is a former scuola and church that’s now a museum; it contains some of my favorite paintings in Venice – the Carpaccio cycle which includes St. George and the Dragon and St. Augustine in his Study (the saint’s dog must be the cutest dog ever painted).
Chorus Pass churches in this sestiere are San Pietro di Castello and Santa Maria Formosa.
CHURCHES IN CASTELLO
Cristo Re alla Celestia
San Francesco della Vigna
San Francesco di Paola (Santi Bartolomeo e Francesco di Paola)
San Giorgio degli Schiavoni
San Giorgio dei Greci
San Giovanni di Malta (Gran Priorale; San Giovanni Battista)
San Giovanni in Bragora
San Lazzaro dei Mendicanti
San Pietro di Castello
San Zaninovo (San Giovanni Novo; San Giovanni in Oleo)
San Zanipolo (Santi Giovanni e Paolo)
Sant'Isepo (San Guiseppe di Castello)
Santa Maria dei Derelitti (Ospedaletto)
Santa Maria del Pianto
Santa Maria della Fava (Santa Maria della Consolazione)
Santa Maria della Pieta (La Pieta)
Santa Maria Formosa
Valdese e Metodista (Chiesa Valdese)
Beata Vergine Addolorata
Ca' di Dio (Santa Maria della Ca' di Dio)
San Marco in Vinea (near Francesco della Vigna)
Santa Maria della Salute (next to Santa Maria Formosa)
I’d love to visit San Lazzaro dei Mendicanti which is inside the hospital complex (formerly the Scuola San Marco, with a gorgeous recently-restored façade). I’ve read that there’s a cat sanctuary in a courtyard inside the hospital which fascinates me too. But I’m not sure how easy or even possible it is to visit – do they really want tourists roaming around their public hospital? UPDATE 11/10: Visited this church!
I’ve tried to visit Santa Maria della Fava at least five times but never found it open. It’s another church with a miracle-working Madonna of some sort. San Zanipolo is one that I want to re-visit; I wasn’t that wowed by it the first time, but I’ve learned a lot about Venice since then so it might be a different experience. It’s an enormous church with the tombs of forty-some doges.
And I want to go check out Sant’Anna, not only because she's my name saint but also because I’m not sure this one is still standing.
Update Dec. 2007: Sant' Anna is there, crumbling and closed, but it IS there. I went inside three new churches in this sestiere: San Martino, Santa Maria della Fava, and San Francesco di Paola.
San Lazzaro remains on my wish list as does Pieta (I think it closes in the winter) and San Lorenzo (still closed but the cats are doing fine!).
I found out that San Giorgio degli Schiavoni is still a consecrated church! The nice man who works there told me that it's primarily a museum but that they do hold Mass there three or four times a year.
Sant' Antonin is deconsecrated and closed. UPDATE: it reopened in 2010 after 20 years of restoration. :)
Discovered two new oratories and added them to that list: San Gioacchino and Ca' di Dio. One source says that they are churches not oratories, but I've put them on the oratory list for now.
Update Dec. 2008. Went inside Sant' Elena. Found out that San Giovanni di Malta is closed for restoration but will re-open at some unknown time in the future.